Why PDFs become large
PDF file size is driven by three main factors: embedded images, embedded fonts, and the number of pages. Images are by far the biggest contributor. A PDF created by scanning a document, for example, is essentially a collection of high-resolution image files wrapped in a PDF container — which is why scanned PDFs are often 10–20× larger than text-based PDFs of the same length.
Fonts add size too, especially when a PDF embeds the full font file rather than a subset. A document using five custom fonts can add several megabytes before a single word of content is counted. Finally, PDFs with many pages accumulate overhead with each page's metadata and structure.
Compression techniques explained
Image downsampling is the most effective compression technique. Most PDFs contain images at 300 DPI or higher — suitable for print, but far beyond what a screen needs. Reducing embedded images to 150 DPI for general use, or 96 DPI for screen-only documents, cuts image data by 50–75% with no perceptible quality difference when viewed on a monitor.
Lossy compression (such as JPEG compression on embedded images) removes image data that the human eye is unlikely to notice. A quality setting of 75–85% typically produces a file that looks identical on screen while being 40–60% smaller.
Font subsetting replaces the full embedded font with only the characters actually used in the document. If your PDF uses Arial but only contains the letters A–Z and 0–9, only those characters need to be embedded. This alone can save hundreds of kilobytes.
Removing hidden data — comments, form fields, embedded JavaScript, and document history — also trims file size without affecting what a reader sees.
When compression reduces quality
Quality loss becomes visible when you compress too aggressively. JPEG compression at below 60% quality produces blocky artefacts, especially around text edges. If your PDF will be printed, keep image DPI at 150 or above. If it contains technical diagrams with fine lines, use lossless PNG compression rather than lossy JPEG.
For documents that will be archived long-term or legally signed, avoid lossy compression entirely. Use lossless techniques only — font subsetting and removing hidden metadata — so the document remains bit-for-bit identical in content.
Step-by-step: compress a PDF online
1. Open the Compress PDF tool.
2. Click or drag your PDF file into the upload area.
3. Select your compression level: Screen (smallest file, lower image quality), Ebook (balanced — best for most uses), or Print (higher quality, modest size reduction).
4. Click Compress PDF and wait a few seconds.
5. Download your compressed file and compare the sizes. Most PDFs shrink by 40–70%.
The original file is never modified. You always get a new compressed copy.
Realistic size reductions to expect
A scanned 10-page document at 300 DPI might start at 8 MB and compress to under 1.5 MB — an 80% reduction. A text-heavy report with a few stock photos might go from 4 MB to 1.2 MB. A presentation-style PDF with full-bleed images might reduce from 15 MB to 5 MB.
Text-only PDFs (generated from Word or Google Docs) are already small and compress less dramatically — perhaps 10–30% — because there is little image data to remove.
Summary
Compressing a PDF is one of the most useful document tasks you can do in seconds. For everyday sharing and email, the Ebook compression level hits the right balance of small file size and retained visual quality. For print-ready files, go conservative. For archiving, stick to lossless. With the right approach, you can cut file sizes by half or more without your recipients noticing any difference.