PDF Compression Explained: Lossless vs Lossy, DPI, and Quality Settings

PDF files grow large for predictable reasons: high-resolution images, embedded fonts, and redundant data left behind by the program that created them. Compression tackles each of these, and understanding how helps you pick the right trade-off between file size and quality.

Lossless vs lossy compression

Lossless compression removes redundancy without discarding any information — text, vector graphics, and structure stay pixel-perfect. It is ideal for documents that are mostly text or line art.

Lossy compression reduces image data by discarding detail the eye is unlikely to notice (similar to how JPEG works). It produces much smaller files but should be used carefully on documents containing fine detail or text-in-images.

Why DPI matters

DPI (dots per inch) controls how much image data each page stores. A scan at 600 DPI looks crisp but is enormous; downsampling to 150 DPI is usually indistinguishable on screen and cuts size dramatically. For print, keep 300 DPI; for screen-only sharing, 96–150 DPI is plenty.

Font subsetting

Embedding only the glyphs a document actually uses — rather than an entire typeface — can shave hundreds of kilobytes from text-heavy PDFs without any visible change.

Choosing a preset

• High quality — light compression, best for documents you may print.
• Balanced — moderate downsampling, the right default for email and web.
• Maximum compression — aggressive lossy settings for the smallest possible file.

Our Compress PDF tool applies these techniques privately, so your file never leaves your device.