Tool guide
Why is my PDF so large?
You photographed five pages, made a PDF, and now it is 20 MB and the upload form says no. The reason is simple, and so is the fix.
What is inside a photo PDF
A PDF made from photos isn’t a typed document. Each page is a picture of the paper. The size of the file is mostly the sum of those pictures, and little else.
A modern phone camera records around 12 megapixels or more, such as 4,032 by 3,024 pixels. Saved as JPEG, that is commonly 2 to 5 MB, and busy detail like paper texture makes it bigger. Five of those pages make a PDF of 10 to 25 MB, however plain the text looks.
What a page really needs
A page doesn’t need all those pixels. What counts is how many pixels land on each inch of paper, called DPI (dots per inch). A4 paper measures 8.27 by 11.69 inches.
| Quality | Pixels needed for A4 | Megapixels |
|---|---|---|
| 150 DPI, fine for reading on screen | 1,240 × 1,754 | about 2.2 |
| 300 DPI, good for printing | 2,481 × 3,507 | about 8.7 |
| Typical phone photo | 4,032 × 3,024 | about 12.2 |
Even at 300 DPI, an A4 page uses fewer pixels than a phone photo holds. At 150 DPI it needs about a fifth. Extra pixels make the file bigger without making the page easier to read.
There is one catch: a photo of a page isn’t a perfect scan. Some of the frame is background, and the text may fill only part of it, so the useful resolution is lower than the number of pixels suggests. That is why the photo should fill the frame, as described in how to photograph documents so they’re readable.
The Make the PDF smaller option
The photos to PDF tool has a tick box called Make the PDF smaller. When it is on, every photo is reduced to 2,000 pixels on its long side and saved as JPEG at quality 0.8 before it goes into the PDF.
Two thousand pixels across an A4 page’s 11.69 inches is about 171 DPI, between the two rows above. That keeps ordinary print legible while cutting a typical phone photo to a fraction of its size.
When the option is off, JPEG photos that are already the right way up go in exactly as they are, with nothing recompressed. That keeps full quality and also keeps the full file size. Other photos are re-saved as JPEG at quality 0.9 and are not reduced in size, except very large ones that are scaled down to keep the tool’s memory use in check.
A good habit is to make the PDF both ways. Use the small one for email and forms, and keep the full one for your records if you need it. If the small print is hard to read in the small version, go back to the full one.
Other things that make a PDF big
Not every large PDF comes from photos.
- Many images. A document with logos, charts and screenshots carries each of them.
- Embedded fonts. To look the same everywhere, a PDF can carry copies of the fonts it uses. A few fonts are small, but a document using many of them grows.
- Scans of whole pages. A scanned page is an image of text, so it is far larger than the same page typed.
- Hidden extras. Some PDFs keep editing history, thumbnails or attachments.
The photos to PDF tool writes pages only, with no editing history, and strips each photo’s location and camera details.
Getting under an upload limit
Email services and upload forms often set a size limit on attachments, from a few megabytes to a few dozen, and it differs from one to the next. Check the limit on the one you are using. Then:
- Tick Make the PDF smaller and build again.
- Reduce the number of pages if some aren’t needed, with the remove button.
- If a single photo is still large, shrink it first with the compress image or resize image tool and add it again.
Questions
How many pixels does an A4 page need?
A4 is 8.27 by 11.69 inches. At 150 DPI that is about 1,240 by 1,754 pixels, and at 300 DPI about 2,481 by 3,507. Most phone photos have more pixels than either.
What does the Make the PDF smaller option do?
It reduces each photo to 2,000 pixels on its long side and saves it as JPEG at quality 0.8 before building the PDF. That is still sharp for reading and for printing at A4.
Will a smaller PDF look worse?
Slightly, if you zoom in a long way. At normal reading size and at A4 print size, text should stay clear. If small print becomes hard to read, rebuild the PDF without the option.
Why is a PDF I received large even though it is mostly text?
Scanned pages are pictures of text, not typed text. Other common causes are embedded fonts and many images inside the file.
What size limit should I aim for?
It varies by email service and upload form, so check the one you are using. Many cap attachments at a few megabytes to a few dozen. Making the PDF smaller first avoids rejections.