A 3-page text-only contract might be 50KB, while another 3-page PDF with a few photos in it comes out to 15MB — the gap looks extreme, but the reasons behind it are pretty straightforward.

The biggest variable: images, and their resolution

Plain text in a PDF is stored as text objects, which is very compact; images are stored pixel by pixel, and the higher the resolution and the more images, the faster file size grows. A single 300dpi high-resolution scanned image can be hundreds of times the size of a whole page of plain text.

Scanned PDFs are usually much bigger than natively generated ones

A PDF exported straight from Word or a web page keeps its text portion tiny; a scan is fundamentally "a photo of every page," so even with identical content, a scanned version is often dozens of times larger than the native one. That's exactly why a scanned copy of the same contract often feels "mysteriously huge" when someone sends it to you.

Font embedding adds some size too, but the effect is limited

To guarantee text renders consistently on any device, PDFs usually embed the font data actually used (especially for CJK text, whose character set is much larger than Latin text's). Modern tools typically only embed the characters actually used (subsetting), so the size increase is usually tens to a few hundred KB — nowhere near as significant as images.

How to tell if a PDF is actually oversized

Does editing make a file bigger?

Editing a PDF with LocalPDF doesn't re-compress existing images or text you didn't actively edit — size stays essentially the same. Only newly added images, a lot of new annotations, or newly used CJK characters will grow the file modestly, purely from that new content itself — not from any quality loss or unexpected bloat (see our article on whether download reduces quality).

Now that you know what drives file size, edit with a clearer picture.

Open LocalPDF →