Why PDFs get so large
A PDF is a container of objects: text streams, fonts, embedded images, vector drawings, and metadata. The two things that almost always dominate file size are embedded images (especially photos and scans) and embedded fonts. A 10-page PDF that is purely text typically weighs under 200 KB; the same number of pages with full-page scanned images can easily exceed 30 MB.
Two compression strategies
PDF compression splits cleanly into two strategies. Lossless cleanup removes metadata, deduplicates fonts, recompresses streams with better algorithms, and strips unused objects — typically saving 10–30% with zero visual change. Image rasterisation goes further: it re-renders each page as a JPEG at a chosen quality, then rebuilds the PDF around those flat images. Rasterisation can shrink an image-heavy PDF by 80% or more, but it permanently loses selectable text and the ability to copy-paste from the document.
When to use each
Use lossless cleanup as your default — it's safe, reversible in spirit, and preserves everything that matters. Use rasterisation only when you've already tried lossless and the file is still too big for your destination, and when the document is being shared as a final read-only artifact (a printed copy, an archived report) rather than a working document. Never rasterise contracts, forms with fillable fields, or anything you'll need to OCR later.
How Converter.Plus compresses PDFs
Our /compress-pdf tool runs entirely in your browser using pdf-lib. Lossless mode strips XMP metadata, removes unused objects, and re-encodes streams. Rasterise mode opens each page in pdfjs-dist, renders it to a canvas at your chosen DPI and JPEG quality, then assembles a new PDF from those rasterised pages. Both modes happen on your device — your contracts and personal documents never travel over the network.
Targeting specific size limits
If you need a PDF under 25 MB for Gmail or under 10 MB for an upload form, start with lossless mode and check the result. If it's still too large, drop to rasterise mode at 150 DPI / 80% quality and try again. For purely-photo PDFs (scans, signed contracts), 100 DPI / 70% quality is usually still legible and yields massive savings. Always open the result and skim a few pages to confirm text is still readable before sending.
Frequently asked questions
Will compressing a PDF lose text searchability?
Lossless mode preserves every text stream. Rasterise mode flattens each page to an image, so you'll lose searchability and copy-paste — but you can run the resulting PDF through OCR if you need text back.
Is it safe to compress signed PDFs?
Compressing a digitally-signed PDF will invalidate the signature. If signature integrity matters, send the original or get the signature reapplied after compression.