My PDF opens and looks fine. How can the page size be wrong?

Because a viewer scales the page to your window, so every page looks the same size on screen regardless of its actual dimensions. Looking right is not evidence. Open the document properties and read the page size in millimetres or points: A4 is 210 by 297 mm, or 595 by 842 points. If the properties say something like 1411 by 1058 millimetres, the file is a poster that happens to have your document printed on it.

Should I use my phone's built-in document scanner instead?

If it has one, yes, for the capture. Those features detect the page edges, correct the perspective and flatten the lighting, which handles the three photographic problems in this article automatically and better than most people manage by hand. Check what comes out, though: some produce a PDF at a real paper size and some hand you an image, and if it is an image you are back to the page-size question the moment you convert it.

Is a photograph of a document acceptable, or do they want a real scan?

Almost always acceptable, and increasingly expected, because the assumption that everyone has a scanner stopped being true. What gets a photograph rejected is not that it is a photograph but that it is a bad one: taken at an angle, half in shadow, cropped through a corner, or too dark to read a stamp. A flat, evenly lit, straight-on shot at a sensible resolution is indistinguishable from a scan for every practical purpose. Where the rules genuinely demand a certified copy, no photograph of any quality will do — that is a different requirement entirely.

Colour or black and white?

Colour, for anything official. A stamp, a signature in blue ink, a hologram on an identity card and a coloured security thread all carry information that says the document is genuine, and removing the colour removes the evidence. Grey is a reasonable saving on a plain black-and-white page, typically a third to a half of the file size, and it should be a deliberate choice rather than a default you did not notice.

The file is too big. Should I lower the resolution or compress it?

Lower the resolution, and do it on the photograph before converting rather than on the PDF afterwards. A PDF built from photographs is images all the way down, so structural compression has almost nothing to remove — the megabytes are in the pixels, and the only way to have fewer megabytes is to have fewer pixels. Reduce once, to the resolution you actually need, and rebuild from the original photograph rather than re-reducing a copy you already reduced.

Photographing a Document and Getting a PDF That Is Accepted

Published 6/30/2026 · 9 min read · File tools

Daniel Okonkwo — Front-end developer and tech writer at Allin

Web performance · File formats

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In short

Shoot from directly above in even daylight, then make sure the PDF you produce is an A4 page rather than a page the size of your photograph. That second step is the one that quietly gets files rejected, and the reason is a unit. PDF measures pages in points, and a point is one seventy-second of an inch. If a converter treats each pixel of your image as one point — which is the simplest thing it can do, and what many do by default — then a 4000 by 3000 photograph becomes a page 4000 by 3000 points, which is 55.6 by 41.7 inches, or 141 by 106 centimetres. It opens, it looks perfectly normal on screen because the viewer scales it to the window, and it is a poster. A4 is 595 by 842 points. A system that expects A4 will either refuse it or scale it in a way nobody predicted, and a person who prints it gets one page of a document spread over a wall chart. So: check the page dimensions of the result, and use a converter that fits the image into a real paper size. On the photograph itself, four things do almost all the work. Shoot from directly overhead, because an angle turns a rectangle into a trapezium and no amount of resolution fixes perspective. Use diffuse daylight and never the flash, which produces a bright disc in the middle of the page. Fill the frame with the document and nothing else. And do not push the resolution: 200 dpi across the page is plenty, which for an A4 sheet means roughly 1650 by 2340 pixels — far less than your phone produces by default, and the difference is weight nobody needs.

Nobody owns a scanner any more, so the phone does it. Two things decide whether the result is accepted: how you took the photograph, and what page size it becomes. The second one surprises people, because a photograph turned into a PDF is not A4 unless something makes it A4.

A point is not a pixel, and that is the whole problem

PDF inherited its measurement from typesetting: pages are described in points, and a point is defined as one seventy-second of an inch. It is a unit of physical length, fixed for all time, and it has no relationship to the pixels in a photograph. A pixel is not a size at all — it is a sample, and how large it is depends entirely on how many of them you decide to spread across a given distance.

So when a converter is handed a photograph and asked to make a page, it has to decide how many points across the image should be. Reading the pixel count as a point count is the least effortful answer and it produces absurd results at phone resolutions: a twelve-megapixel image is around 4000 pixels wide, which read as points is 4000 divided by 72 inches — over a metre and a half. The alternative is to pick a paper size, scale the image to fit inside it and centre it. Both are one line of code. Only one of them produces a document.

Why nobody notices until it is too late

A PDF viewer scales any page to the window, so a page a metre and a half wide and a page of A4 look identical on screen: the same document filling the same rectangle. Nothing in the interface announces the dimensions unless you open the document properties and look. The mismatch only becomes visible at the moment it costs something — when a portal validates the upload, when a case worker prints the file, or when a system tiles your one page across six sheets of paper.

The check takes five seconds and is worth making into a habit. Open the finished PDF, look at the document properties, and read the page size. A4 reads as 210 by 297 millimetres or 595 by 842 points; US Letter as 8.5 by 11 inches or 612 by 792 points. Anything else, and you are about to send a poster.

The photograph: four things, in order of impact

Angle first, because it is the one that cannot be repaired. Photograph a rectangular page from off to one side and it arrives as a trapezium — the far edge shorter than the near one, the lines of text converging. Text recognition struggles with it, a reviewer reads it as sloppiness, and no increase in resolution helps, because the geometry is wrong rather than the detail insufficient. Standing over the page costs nothing and removes the problem entirely.

Then light. Diffuse daylight from a window is the best source most people have, and the flash is the worst, because it fires from a few centimetres away and leaves a bright disc in the middle of the page with darkness at the corners. Watch for your own shadow, which is the commonest ruin of an overhead shot: if it falls across the paper, turn the paper rather than leaning further. Then framing — fill the viewfinder with the document, since pixels spent on your table are pixels the recipient pays for in file size. And finally resolution, which almost nobody needs to increase: 200 dpi across an A4 sheet is about 1650 by 2340 pixels, comfortably under four megapixels, and your phone is probably producing three times that.

Several photographs, one document

A multi-page document photographed page by page should arrive as one PDF with pages in order, not as six attachments named IMG_4471 onwards. Convert them together in one pass so the ordering is decided once, and name the files with two-digit prefixes first if the converter takes them in file order — the same sorting trap that scrambles merged packs applies here, for exactly the same reason.

What the photograph carries besides the document

A phone photograph carries metadata, and depending on your settings that can include the coordinates where it was taken. Photographing your passport at the kitchen table and sending the result to a letting agent can hand them your home address in a field nobody looks at. Converting to PDF does not reliably remove it — the image is embedded, and what travels with it depends on the converter. If the document is sensitive, strip the metadata from the photographs before converting, or check the finished PDF rather than assuming.

Where a photograph's pixels end up, depending on how the converter sizes the page
Photograph	Page = pixels as points	Page = fitted to A4	Effective resolution on A4
1650 × 2340 (4 Mpx)	58 × 83 cm	21.0 × 29.7 cm	200 dpi — the sensible target
4000 × 3000 (12 Mpx)	141 × 106 cm	21.0 × 29.7 cm	Over 340 dpi — more than anyone needs
8000 × 6000 (48 Mpx)	282 × 212 cm	21.0 × 29.7 cm	Wasted — the file is heavy for nothing

Image to PDF (JPG, PNG, WebP, GIF, BMP, AVIF, TIFF)Combine JPG or PNG images into a single PDF, one image per page. WebP, GIF, BMP, AVIF and TIFF are converted on the way in.Try the tool →

Frequently asked questions

My PDF opens and looks fine. How can the page size be wrong?: Because a viewer scales the page to your window, so every page looks the same size on screen regardless of its actual dimensions. Looking right is not evidence. Open the document properties and read the page size in millimetres or points: A4 is 210 by 297 mm, or 595 by 842 points. If the properties say something like 1411 by 1058 millimetres, the file is a poster that happens to have your document printed on it.
Should I use my phone's built-in document scanner instead?: If it has one, yes, for the capture. Those features detect the page edges, correct the perspective and flatten the lighting, which handles the three photographic problems in this article automatically and better than most people manage by hand. Check what comes out, though: some produce a PDF at a real paper size and some hand you an image, and if it is an image you are back to the page-size question the moment you convert it.
Is a photograph of a document acceptable, or do they want a real scan?: Almost always acceptable, and increasingly expected, because the assumption that everyone has a scanner stopped being true. What gets a photograph rejected is not that it is a photograph but that it is a bad one: taken at an angle, half in shadow, cropped through a corner, or too dark to read a stamp. A flat, evenly lit, straight-on shot at a sensible resolution is indistinguishable from a scan for every practical purpose. Where the rules genuinely demand a certified copy, no photograph of any quality will do — that is a different requirement entirely.
Colour or black and white?: Colour, for anything official. A stamp, a signature in blue ink, a hologram on an identity card and a coloured security thread all carry information that says the document is genuine, and removing the colour removes the evidence. Grey is a reasonable saving on a plain black-and-white page, typically a third to a half of the file size, and it should be a deliberate choice rather than a default you did not notice.
The file is too big. Should I lower the resolution or compress it?: Lower the resolution, and do it on the photograph before converting rather than on the PDF afterwards. A PDF built from photographs is images all the way down, so structural compression has almost nothing to remove — the megabytes are in the pixels, and the only way to have fewer megabytes is to have fewer pixels. Reduce once, to the resolution you actually need, and rebuild from the original photograph rather than re-reducing a copy you already reduced.

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