What makes a PDF large in the first place
Three things typically drive file size:
- Images — scanned pages and embedded photos are by far the biggest contributor. A single high-resolution scanned page can be several megabytes on its own.
- Embedded fonts — PDFs often bundle full font files so the document displays identically everywhere, which adds size but is rarely worth removing.
- Redundant data — duplicate objects, unused metadata, or bloated internal structure from repeated editing.
The compression that doesn't cost you quality
Removing redundant internal data and optimizing how the file is structured reduces size with zero visible change — this is the "free" part of compression, and a good compressor applies it first.
The compression that involves a tradeoff
Beyond that, real size reduction on image-heavy PDFs comes from reducing image resolution or increasing JPEG compression on embedded photos. This does involve a quality tradeoff, but a small one is usually invisible on screen and only becomes noticeable if you zoom in far past normal reading size, or print at large formats.
Practical settings to think about
- For screen reading and email: images at 150 DPI are typically indistinguishable from the original on a monitor, and cut file size dramatically compared to a 300+ DPI scan.
- For printing: keep resolution higher (300 DPI is the common print standard) — this is where visible quality loss is most likely if you compress too aggressively.
- For text-only PDFs: compression has little to gain, since text itself takes up very little space compared to images.
A quick way to check if compression went too far
After compressing, open the result and zoom in on any photo or scanned text at 100–150%. If it's still crisp and readable, the compression was safe. If it's noticeably blurry or pixelated, dial back the compression level and try again.
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