If a PDF barely shrinks when you compress it, it's almost always one of two reasons: it's mostly digital text (there was little image data to squeeze) or it's already been compressed once. Here's how to tell which you have, and what actually reduces the size when compression alone won't.
Reason 1: it's mostly text
A PDF's size is dominated by its images. A file that's mostly digital text was already small to begin with — there's little to compress, so a compressor can only shave a few percent. That's completely normal. The big savings come from scanned or photo-heavy PDFs, not text documents.
Reason 2: it's already compressed
If a PDF was compressed once, the redundant image data is already gone. Running it through a compressor again barely changes it — you can't squeeze the same data twice. If a file hardly moves, it may simply have been optimised before you got it.
How to tell which one you have
Scroll through the PDF. If the pages look like photos of paper — a scan — it's image-heavy and should compress well; if it doesn't, it was probably already compressed. If the pages are crisp, selectable digital text, it's text-based and simply won't shrink much no matter what you do.
What actually reduces the size then
- Remove pages you don't need with Delete Pages or Split PDF — fewer pages, smaller file.
- If it's a scan you only need the text from, use Image to Text OCR — far smaller than page images.
- If you have the source document, re-export the PDF at a lower quality from there.
Start by running it through Compress PDF to see how much you can actually save — it works in your browser, so the file never leaves your device. Trying to hit a specific limit? See how to reduce a PDF below 5 MB.