A bloated PDF is almost always a symptom of one thing: uncompressed images embedded at a much higher resolution than the document actually needs. Understanding this makes compression much less mysterious.
Where the size actually comes from
Text and vector graphics in a PDF take up very little space — a hundred-page text-only document might only be a few hundred kilobytes. Photos and scanned pages are a different story. A single high-resolution photo embedded at print quality can be several megabytes on its own, and a scanned document is really just a series of full-page photos.
How compression works without ruining quality
Good compression re-encodes embedded images at a resolution and quality level appropriate for on-screen viewing, which for most documents is far lower than what a scanner or camera originally captured. The text layer is left untouched, since it was never the source of the bloat in the first place.
Practical tips
- If a PDF is mostly text with a logo or two, its size should already be small; heavy compression will not help much.
- For scanned documents, moderate compression can cut file size dramatically with no visible quality loss on a screen.
- Always compare the compressed file against the original before deleting the original — keep a backup until you are sure the result works for your purpose.
- If a document needs to be printed at high quality later, keep an uncompressed copy for that purpose.
Have images to shrink instead?
Open Compress Image