Why Is My PDF File So Large? (And How to Fix It)

The most common cause: images, not text

Text and vector graphics take up almost no space in a PDF — a hundred pages of plain text can weigh less than one megabyte. Large file sizes almost always come from embedded images: scanned pages saved at a high resolution, screenshots pasted in at full size, or photos placed without any compression.

A quick way to check: if a PDF came from scanning paper documents or exporting from a phone camera, images are almost certainly the reason it’s large.

What structural compression can and can’t do

Some compression tools rebuild the PDF’s internal structure — removing duplicate objects, optimizing how fonts and streams are stored. This helps, but has a ceiling: it doesn’t touch the images themselves, so a file that’s 90% image data won’t shrink much no matter how well the structure is optimized.

For real gains on image-heavy files, the images need to be re-encoded at a lower resolution or higher compression before they’re placed back in the PDF.

A practical fix

If a PDF is mostly text and diagrams, structural compression alone is often enough. If it’s mostly scans or photos, consider re-scanning at a lower DPI, or converting the pages to images, compressing those, and rebuilding the PDF.