Email attachment limits, slow uploads, storage caps — PDF file size becomes a problem the moment you need to actually send a document rather than just keep it on your own device. Compression solves this, but not all compression is equal.
What's actually being compressed
Most large PDFs are large because of embedded images — scanned pages, screenshots, or high-resolution photos. Compression works primarily by reducing image resolution and re-encoding it more efficiently. Text and vector content barely change in size regardless of compression level, since text is already stored efficiently.
Choosing a compression level
- Light compression — best when the PDF will be printed or contains detailed diagrams. Minimal visible quality loss.
- Medium compression — the right default for most email attachments and shared documents. Noticeably smaller with quality that holds up on screen.
- Strong compression — use only for archiving or when file size is the absolute priority over visual fidelity, such as bulk scanned records.
What compression won't fix
If a PDF is large because it contains dozens of full pages of embedded fonts or has been scanned at unnecessarily high DPI, compression helps, but the better fix is scanning at a lower resolution to begin with, or removing unused embedded fonts before compressing.
A simple rule of thumb
Start with medium compression for anything you're emailing. Only drop to strong compression if the file still doesn't meet a size limit, and only go lighter than medium if the document is going to be printed professionally or contains content where fine detail actually matters, like architectural drawings or medical scans.