Compress PDF
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Why PDF File Size Matters — and How to Choose the Right Compression
Most people only notice PDF size when it’s too late: an email that won’t send, a website upload that times out, or a document that takes forever to download on mobile. The truth is that file size is one of the most overlooked factors in document sharing, and choosing the right compression level can save you hours of frustration without sacrificing the quality you need.
When does a PDF become too large?
Email providers typically cap attachments at 20–25 MB. Many business portals, job application systems and government sites reject anything over 10 MB. If your scanned PDF is 15 MB, you’re not just “using a bit more space” — you’re hitting hard limits that prevent the file from being accepted at all. Even when the file goes through, a 20 MB download on a 3G connection takes over a minute, and most recipients will give up before it finishes.
Standard vs Deep compression — what’s the difference?
Standard compression works by optimising the internal structure of the PDF: re-compressing embedded images, removing redundant metadata and streamlining the font tables. It’s fast, lossless in terms of text and vector graphics, and typically reduces file size by 10–30%. This is the right choice for text-heavy documents, contracts and invoices where every word must stay crisp.
Deep compression goes further: it rasterises each page into an image, then re-encodes it at a lower DPI and quality level. This is what shrinks a scanned photo-PDF from 20 MB down to under 2 MB. The trade-off is that text becomes part of the image — you can no longer select or copy it — and very small text may look slightly softer. For scanned documents, receipts and photos that only need to be readable, this is almost always the better choice.
Which settings should you pick?
For email attachments: aim for under 5 MB. Use Deep compression at 150 DPI with 70–80% quality. Text remains perfectly readable, and the file will send through any email provider.
For web uploads (job applications, forms): under 2 MB is ideal. Drop to 100 DPI and 60–70% quality. These portals only need the document to be legible on screen — they don’t print it.
For archival or printing: stick with Standard compression, or use Deep at 300 DPI with 90% quality. You want the text sharp enough to print at 100% size without blur.
Why compress in your browser instead of uploading to a server?
Most online PDF compressors send your file to their servers, process it there, and hand it back. That’s fine for a casual photo, but it’s a real privacy risk for documents containing ID numbers, signatures, financial data or medical information. When you compress a PDF on PixlReady, the file never leaves your device — the compression happens locally in your browser using WebAssembly. You can verify this by opening your browser’s developer tools and watching the network tab: there are no upload requests.
Need to do more with your PDF? Try our merge PDF, split PDF or add page numbers tools — all processed locally, no upload required.
Standard vs deep compression: when each one actually helps
Most PDFs balloon for one of two reasons. Either the file is a born-digital document whose internal objects grew lazy over time — repeated saves, embedded thumbnails, unused font subsets — or every page is already a full-page photograph or scan, and the JPEGs inside it were saved at near-maximum quality. The two modes on this page map exactly onto those two causes, and choosing the wrong one is why people think a PDF compressor "did nothing".
Standard compression loads the document with pdf-lib and saves it back with optimized object streams, cross-reference streams and deduplicated structure. It never touches a rendered page: vector drawings stay vector, embedded fonts stay embedded, and text remains selectable, searchable and copyable. On a born-digital Word export it typically shaves ten to thirty percent with zero visible change, and it is instant — there is no per-page rendering.
Deep compression ignores the existing structure and rebuilds the file from scratch. Each page is rendered by pdf.js at the DPI you choose, encoded as a JPEG, and placed into a fresh one-page-per-slot PDF. This is the mode that actually shrinks a 40 MB scan-to-PDF: the renderer discards the huge embedded image, and the new JPEG at 120 DPI and quality 0.7 is dramatically smaller while remaining perfectly readable on screen and on a home printer.
- Pick Standard for: contracts, resumes, invoices, reports, slides exported from Keynote or PowerPoint — anything where you can highlight a word and copy it.
- Pick Deep for: scanned contracts, faxed paperwork, phone photos of receipts, and PDFs where selecting "text" actually selects a whole page image.
- Trade-off to remember: after Deep mode, the page is a picture. Highlights, copy-paste and text search are gone — and that is expected, not a bug.
- If the result is still too big, lower the render DPI to 96 or the JPEG quality to 0.5; if banding appears in photos, raise quality toward 0.85.
How the result is measured, and where PDFs fit in your pipeline
Right after compression the page shows three numbers: the original size, the new size, and the percentage saved. The percentage is computed from the byte count of the file you actually download, so it matches what your email attachment meter or cloud upload bar says. Standard mode rewrites the file once in memory and the savings report arrives immediately; deep mode shows a live progress bar because every page has to be rasterized, and a 50-page scan takes a few seconds longer than a one-pager.
Compression is usually the last step in a PDF workflow, not the first. If your file came from a stack of separate scans, merge the PDFs first. If you only need a few pages out of a big deck, split the PDF before compressing so you do not pay to shrink pages nobody reads. And when the goal is to quote a paragraph or reuse a diagram, turn the pages into images instead — then run the exported JPEGs through the image compressor for the smallest possible web-friendly set.
- Files are read into the browser as ArrayBuffers and written back as a downloadable blob; nothing is streamed to a remote server.
- The downloaded file is named yourfile-compressed.pdf so you never overwrite the original — keep both until you have opened the result.
- Password-protected PDFs are rejected up front rather than half-processed; unlock the file in its viewer first and try again.
Frequently Asked Questions
How does this PDF compressor work?
Standard mode rebuilds the PDF structure with optimized object streams, which shrinks most text PDFs without touching one pixel of the page. Deep mode renders every page to a JPEG and rebuilds the document around those images, which is what works on scanned and photo PDFs.
Is my PDF uploaded when I compress it?
No. Compression runs entirely in your browser with pdf-lib and pdf.js. The file never leaves this device and nothing is queued on a server.
Which mode should I pick?
Start on Standard. It is fast, lossless in appearance, and keeps the text selectable and copyable. Switch to Deep only when Standard barely moves the needle — that happens mostly with scans and PDFs that are already full-page images.
Why does deep compression make the text unselectable?
Deep mode turns each page into a JPEG raster. After that the page is a picture, so words cannot be highlighted or copied — by design. It is the same reason a scanned PDF cannot be searched.
What do DPI and JPEG quality do in deep mode?
DPI is the render resolution: 96 for screen sharing, 120 balanced, 150 when you still want a clear print. JPEG quality trades file size against visible blockiness; 0.7 is the balanced default and 0.85 keeps the most detail.
Does compression remove the password or let me edit the PDF?
No. Encrypted or password-protected files cannot be opened by the local engine and are rejected with a clear message. Compression is not editing — it only repackages or re-encodes the existing pages.