PDF Compression: What It Can (and Can't) Do Yet

"Compress this PDF" sounds like one simple operation, but what actually happens under the hood - and how much smaller the file gets - depends heavily on what's inside it.

Where the size actually comes from

A PDF's file size generally comes from two very different sources: its internal structure (how many objects, fonts, and cross-references it contains) and its embedded images. A text-heavy document with a lot of small internal objects can often be meaningfully shrunk just by re-serializing it more efficiently - removing redundant structure without touching a single pixel of visible content. A PDF that's mostly high-resolution scanned images, on the other hand, carries almost all of its size in those images - and no amount of restructuring the document's internal object graph will make a meaningful dent in that.

An honest limitation in our own tool, right now

Our Compress PDF tool is built on pdf-lib, a well-regarded library for reading and writing PDF structure - but as of today, it can't recompress or downsample embedded images. That means every compression profile (Low, Balanced, High, Maximum) currently produces the same output: the structural optimization runs regardless of which one you pick, but none of them touch image data yet.

In practice, this means:

• Text-heavy PDFs (reports, contracts, forms) often see a real, worthwhile size reduction.
• Image-heavy PDFs (scanned documents, photo-heavy brochures) see comparatively little change, since their size lives almost entirely in image data the tool doesn't yet touch.

Per-profile image compression - actually recompressing and downsampling embedded images at different quality levels per profile - is planned for a future update. We'd rather tell you this directly than have the profile selector imply it's doing something it isn't.

What to do in the meantime

If your PDF is mostly scanned pages or photos and you need a real size reduction, compressing the source images before building the PDF (or converting pages to a compressed image format via our PDF to Image tool, if you don't need it to stay a PDF) will get you further than any PDF-level structural optimization can today.

Try it yourself

Compress PDF is free, has no sign-up requirement, and shows you the exact before/after size and percentage reduction so you can see for yourself whether a given file benefited.