PDF Glossary: Compression, DPI, Optimization, and File Size
Plain-language definitions for PDF compression terms including DPI, downsampling, lossy, lossless, optimization, and metadata.
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DPI
Dots per inch. In PDF compression, DPI often describes image or scan resolution. Higher DPI can improve print detail but increases file size.
Downsampling
Reducing image resolution inside a PDF. It is one of the most effective ways to shrink scanned or photo-heavy documents.
Optimization
A cleanup process that removes unnecessary data, rewrites objects efficiently, and prepares the PDF for smaller storage or faster transfer.
Lossy and Lossless
Lossy compression discards some visual data. Lossless compression preserves source data. See lossy vs lossless PDF compression.
Where to Go Next
For the complete topic map, start with the PDF compression guide. When you are ready to reduce a file, open Compress PDF and export a smaller copy in your browser.
If your output is still too large or looks blurry, use the compression troubleshooting guide to narrow down the cause.