Compression is most useful for a scanned PDF whose pages are already images. A text-heavy PDF may already be compact and may not become smaller. FileOar renders every page as an image, adjusts resolution and JPEG quality, then builds a new PDF. If that PDF is larger, the original file is returned instead.
Open Compress PDF, select one PDF and preview its first two pages. Choose a resolution and image quality, then run compression. Lower settings can save space but may make small text difficult to read. Compare the reported file sizes, download the result and check pages throughout the document, especially fine print, signatures, tables and diagrams.
A newly compressed PDF loses selectable and searchable text, clickable links, form fields and annotations because its pages become images. Transparent regions also become white. If those features matter, keep the original PDF or use a different workflow that preserves them. An unchanged result is the original file, so its existing features remain.
The tool does not support encrypted PDFs and limits each document to 200 pages and 80 million rendered pixels. Break a large document into smaller parts before trying again if it exceeds those limits.