Why is my PDF so big? What fills it and what compression can do

Photos at camera resolution

Images are generally the biggest cause of large PDFs. When a photo goes into a document at the resolution the camera took it, the file carries every one of those pixels, even if the photo appears only a few centimetres wide on the page. The page looks the same either way; the file doesn't.

Scanned pages

A scanner or phone scanning app usually turns each page into a picture. The text you see is part of that picture rather than real characters, so every page is an image. A 20-page scan is 20 images, which is why scanned PDFs are often far larger than PDFs made from a word processor.

Embedded fonts

A PDF can carry the fonts it uses, so it looks the same on every device. A complete font can be large compared with the text itself. A tutorial on Acrobat's optimiser notes that embedded fonts can take a very high share of a small document, sometimes 30 percent or more. Embedding only the characters actually used (a subset) keeps this down.

Saved again and again

Some PDF editors save changes incrementally: they add the changes to the end of the file and never delete the old data. Each save is quick, but the file keeps growing, and content you deleted can still be inside. A full save that rewrites the whole document drops the leftovers, which is why simply re-saving a PDF in a different app can sometimes make it smaller.

What compression can and can't change

Compression can only shrink what is shrinkable. Photos can be stored at fewer pixels and a lower JPEG quality. Scanned pages can be redrawn as lighter images. Text, on the other hand, is already tiny: a page of plain text takes very little space, so a PDF that is mostly text barely changes. If such a file is still large, the cause is usually fonts or a long save history rather than the text.

Our tool doesn't remove fonts. On a PDF with no photos, it reports that only the file structure was tidied up, and if the result isn't smaller it keeps the original.

Balanced or Strong: the trade-off

Balanced re-saves the JPEG photos inside at a lower quality and size. Text, links and form fields are left exactly as they are, so the PDF still behaves like the original. It leaves images in CMYK and unusual colour spaces as they are. Use it for reports, brochures and documents with photos.

Strong turns every page into a JPEG image, at 200, 150 or 100 DPI for Less, Recommended or More. It gives the biggest saving on scans, because each page becomes one lighter picture. The cost is real: the text can no longer be selected, searched or copied, and links and form fields are lost. That's fine for a scanned letter you need to email, and wrong for a form someone has to fill in.

Check the result before deleting the original

Open the compressed file and look at it the way the reader will. Zoom in on photos and the smallest print. Check that every page is there. After Balanced, try searching for a word; after Strong, remember that searching won't work. If the receiver needs to fill in a form or click links, use Balanced or send the original. Keep the original until you're sure the smaller copy does its job.