Large PDF files cause problems: email attachments bounce, upload forms reject them, sharing links time out, and storage fills up quickly. PDF compression reduces file size by optimising how content is stored — most visibly by resampling embedded images, which account for the majority of a large PDF's size. This guide explains how compression works and how to get the best results without damaging readability.
How PDF Compression Works
A PDF file contains multiple types of content: text (stored as font data and character codes), vector graphics (stored as mathematical instructions), and raster images (stored as pixel grids). Compression primarily targets raster images because they take up the most space. The compressor reduces image dimensions to match the intended viewing resolution and applies JPEG or other encoding to reduce pixel data. Text and vectors are largely unaffected, which is why compressed PDFs remain sharp for reading even as file size drops significantly.
How to Compress a PDF — Step by Step
- Open the PDF Compress tool. No installation required. The tool runs in your browser.
- Upload your PDF. Drag and drop the file onto the upload area or click to select it from your device.
- Choose a compression level if the option is available. Higher compression produces smaller files but may reduce image sharpness. For documents that will be read on screen, medium compression usually works well. For archiving, use lower compression to preserve quality.
- Click Compress. The tool processes your file locally — no upload to any server.
- Download the compressed PDF. Compare the file size before and after. If the result is not small enough, you can compress again, though diminishing returns apply after the first pass.
How Much Can You Reduce a PDF?
Results vary significantly depending on what is in the PDF. A scan-heavy file (such as a scanned contract or photocopied notes) can compress by 70–85% because the images contain large amounts of redundant pixel data. A text-only PDF — like an exported Word document — may only compress by 10–20% because text is already stored efficiently. A mixed document (some images, some text) typically sees 40–60% reduction.
When Compression is Not the Answer
If a PDF contains high-resolution photos that need to remain sharp for print or professional presentation, aggressive compression may introduce visible artefacts. In that case, consider splitting the document and compressing only the less critical sections, then merging them back together. Alternatively, convert images to a more efficient format before embedding them in the PDF.
Common Mistakes to Avoid
- Compressing an already-compressed PDF repeatedly. Each compression pass degrades quality further with diminishing file size savings. Compress from the original source document, not a previously compressed version.
- Sending the wrong file. Always check the compressed version opens correctly before replacing the original. Keep the uncompressed file as a backup.
- Assuming all PDF compressors are equal. Server-based tools may handle your documents differently and store them after processing. EasyPZ Tools compresses entirely in your browser, so your files remain private.