Sooner or later everyone hits the same wall: an email bounces because the attached PDF is 30 MB, or a government portal rejects your scan for exceeding its 5 MB limit. The good news is that most oversized PDFs can be shrunk dramatically — often by 80–90% — with little or no visible quality loss, once you understand why the file is big.
Why PDFs Get Huge
A PDF is a container. Text and vector graphics inside it are extremely compact — a 300-page novel as pure text is barely 1 MB. What inflates PDFs is almost always images:
- Scanned pages are full-page photographs, often stored at 300+ DPI in lossless formats.
- Embedded photos pasted into reports at full camera resolution (a modern phone photo is 3–8 MB alone).
- Unoptimized exports from design software that embed fonts and images without compression.
Two Kinds of Compression — Choose Deliberately
Structure optimization (lossless). Rebuilds the PDF's internal structure, deduplicates objects and applies stream compression. Text stays selectable and quality is untouched. Savings are modest for already-optimized files (5–20%) but this should always be your first attempt for text-based documents.
Image recompression (lossy). Re-encodes the images inside the PDF at a lower quality or resolution. This is where the dramatic 80–90% savings come from on scanned documents. The trade-off: pushed too far, text in scans becomes fuzzy. A quality setting of 60–75% is usually indistinguishable on screen.
Step-by-Step: Compressing a PDF in Your Browser
- Open the free Onnax PDF Compressor — it runs locally in your browser, so the document is never uploaded.
- Drop in your PDF and choose a mode: Light for text documents you want to keep selectable, Strong for scans and image-heavy files.
- In Strong mode, set image quality. Start at 60%: for a typical scanned contract this cuts a 20 MB file to about 2 MB.
- Download, open, and zoom to 150% to sanity-check the smallest text before sending.
Pro Tips for Smaller PDFs From the Start
- Scan documents at 150–200 DPI grayscale instead of 300 DPI color — perfectly readable at a third of the size.
- Resize photos to their display size before inserting them into documents (our Image Resizer takes seconds).
- Exporting from Word? Choose "Minimum size" in the PDF export options.
- Splitting one giant PDF into parts with a PDF splitter can beat compression when a portal has a hard per-file cap.
Privacy Matters More for Documents
Think about what's inside the PDFs people compress: contracts, medical results, payslips, ID scans. Uploading those to a random server to shave megabytes is a bad trade. Browser-based compression sidesteps the issue entirely — the file never leaves your machine, which is exactly how the Onnax compressor works.
The Bottom Line
Match the method to the document: lossless optimization for text PDFs, image recompression around 60–75% quality for scans. Keep your original, verify the result, and you'll almost never fail an upload limit again.