Why Two PDFs With Similar Content Can Have Wildly Different File Sizes

It's a common surprise: two PDFs, similar page counts, similar-looking content, and one is a few hundred kilobytes while the other is tens of megabytes. Page count turns out to be one of the least important factors in PDF file size, what actually drives it is almost always the content embedded inside those pages.

Text itself is remarkably cheap in terms of file size. A PDF that's purely text, even hundreds of pages of it, compresses extremely well because text has a lot of repetitive structure that compression algorithms handle efficiently. A 300-page text-only PDF might easily stay under a megabyte.

Images are almost always the real culprit. A single high-resolution photo embedded at print quality can easily be several megabytes on its own, and PDFs often contain images that are far higher resolution than needed for on-screen viewing, a scanned document, a photo pasted in at full camera resolution, a screenshot saved without compression. Multiply that across many pages of a scanned document, and file sizes balloon quickly.

Fonts play a smaller but real role too. When a PDF embeds a full font file (so the document displays correctly on any device, even ones without that font installed), that adds overhead, especially if multiple font weights or families are embedded. This is usually a much smaller contributor than images, but it adds up in documents with unusual typography.

This is also why PDF compression results vary so much between different documents. A PDF compression tool can meaningfully shrink a file with large embedded images, sometimes dramatically, but a PDF that's already mostly text and light on images has much less to compress in the first place, there's simply less redundant data to remove. Our Compress PDF tool works entirely in your browser and will show you the before and after size so you can see exactly how much, if anything, was saved for your specific file.