Why is my PDF so large?

2026-10-10

A PDF’s size has very little to do with its number of pages. One page can be 50 KB or 15 MB. What matters is what’s inside.

The usual causes

Scanned pages. Every scanned page is a picture. At 300 DPI in color, a single A4 page can be several megabytes before compression. This is the most common reason for a large PDF — and the one compressing helps most with.

Photos straight from a camera. A modern phone photo has 12 to 48 megapixels. Placed on a page, it still keeps all of those pixels even if it’s shown at the size of a postcard.

The same image saved many times. A logo or background repeated on every page is sometimes stored once and sometimes once per page, depending on the program that made the PDF.

Attached files. A PDF can carry other files inside it — spreadsheets, other PDFs, videos. They count toward the size but aren’t visible on the pages.

Fonts. Embedded fonts, especially for Asian scripts, can add a few megabytes.

Print-color and wide-gamut images from print workflows tend to be large, and they can’t be saved again without shifting their colors.

What compressing can fix

Compress PDF reduces the first three: oversized scans, oversized photos and repeated images (an image that is shared across pages stays shared, so it is stored once). Photos placed inside grouped graphics, as design and presentation tools export them, and transparent images are handled too; transparency is kept. On those files the reduction is usually large.

It doesn’t touch attachments, fonts, print-color images or images with a wide-gamut color profile. When those make up most of a file, the result says so — for example “Most of this PDF is attached files” — instead of pretending.

Find out in one step

Open the PDF in Compress PDF and press Compress PDF with the recommended Balanced level. You’ll either get a smaller file with the exact saving, a message that the PDF is already well compressed, or a short explanation of which parts couldn’t be made smaller. The file never leaves your device.