The Complete Guide to PDF Compression
PDF compression can reduce file size by re-saving PDF structure or reducing image data. The most common reason to do it is to get an oversized scan, ID photo or form under an upload limit (100KB, 200KB, 500KB or 1MB) while keeping it readable. This guide explains compression methods, how to request a target with compress cat, and what to check if a file won't shrink. Processing runs entirely in your browser — your files are never uploaded.
What is PDF compression?
PDF compression tries to reduce file size. Re-saving PDF structure can preserve selectable text; reducing image data may save more space but lose detail. Scans and phone snapshots are bitmaps whose resolution, color and encoding affect their size. Each file in compress cat starts with 'Keep selectable text'; 'Compress as images' explicitly rebuilds pages as pictures.
Text-only PDFs (exported straight from Word, Google Docs or LaTeX) are a different story — the text itself takes up almost no space, so there's little to squeeze. The files that really shrink are image-heavy scans and photo PDFs. Knowing this is the key to judging how small a given file can realistically get.
Lossy vs. lossless compression: what's the difference?
Lossless compression removes redundant data without discarding image detail. Savings depend on the PDF and its existing encoding; there is no fixed percentage or guarantee that it will meet an upload limit.
Lossy compression discards image detail, for example through lower resolution or stronger JPEG compression, for a potentially smaller file. Images and text edges may soften. In compress cat, 'Compress as images' also removes selectable text, search, links and interactive fields.
- Lossless: preserves image detail, but savings vary and an already optimized file may barely shrink.
- Lossy: trades detail for possible size reduction; reaching a 200KB / 500KB / 1MB limit still depends on the file.
- Practical tip: keep the original, choose the method for each file, and check output bytes and every page before submitting.
When do you actually need to compress a PDF?
You almost always need to compress because some system caps the file size. The exact limit varies a lot by context — here are the four most common, each with a dedicated scenario guide you can read for details.
- Exam & test registration: photo and form uploads are often capped at ≤200KB (sometimes ≤50KB for the photo field).
- Visa applications: consulate portals and visa centers commonly require each PDF to be ≤1MB (some allow ≤2MB), across many documents.
- Government & agency portals: business licenses, contracts and filings are frequently limited to ≤500KB, and oversized files are rejected outright.
- Email attachments: Gmail caps attachments at 25MB and Outlook at ~20MB, so large scanned PDFs often need trimming before they'll send.
How do you request a target file size?
compress cat treats a size target as a best-effort goal. Each file defaults to 'Keep selectable text'; changing the target does not switch the method. Choose 'Compress as images' only if you accept losing text selection, search, links and interactive fields, with possible loss of detail. Targets use KiB: 1 KiB = 1,024 bytes and 1 MiB = 1,048,576 bytes. Compare the displayed byte target with the receiving site's actual limit.
- Open Compress PDF and drag your PDFs in, or choose files (up to 5 PDFs, 50 MiB each).
- Set 'Target file size' in KiB or select a preset, then review the method for each file. Keep 'Keep selectable text' unless you explicitly want image compression.
- Click 'Compress files' to process locally. If one file fails, the other files can still finish.
- Review each result's bytes and target status, compare all pages, then choose 'Download output'. An above-target result is best effort, not the smallest possible file; if no smaller version is found, the original is retained.
Why won't my PDF compress? Troubleshooting
A file may shrink little because of its content, existing compression or the selected method. An above-target result does not prove a physical minimum. Check any per-file error as well as the result size.
- Already compressed images may offer little further reduction without losing detail. Try a looser target or improve the source document.
- Many pages can make a small target difficult. If the submission permits it, split the PDF or remove blank and irrelevant pages first.
- The target is too aggressive: crushing a crisp color scan to 30KB usually makes it illegible. Try 200KB first, then tighten in steps.
- Color you don't need: a black-and-white text scan doesn't need color — exporting it as grayscale shrinks it a lot.
- The default 'Keep selectable text' method avoids rasterizing pages. 'Compress as images' turns pages into pictures and removes text selection and search.
Scanned vs. text PDFs: why it matters
Content strongly affects compression. Scans and photo PDFs contain large images, so reducing resolution or quality may save space, but the amount varies. Some scans also have an OCR text layer that image compression would remove.
Text PDFs exported from Word, Google Docs and similar tools often have less image data to reduce. Start with 'Keep selectable text', which re-saves PDF structure without rasterizing pages. Choose 'Compress as images' only if a smaller output matters more than selectable text and you accept possible detail loss.
- Scans / photo PDFs: image compression may reduce size, but inspect small print and any OCR text requirements.
- Pure-text PDFs: start with 'Keep selectable text'; savings may be small, and the original may already fit.
- Mixed PDFs (text + big images): start with 'Keep selectable text', or reduce source images before exporting again. Image compression loses selectable text.
Frequently asked questions
Is a 200 KiB output guaranteed?
An exact size is not guaranteed. Request a target, then check the output's byte count and target status. A 200 KiB target is 204,800 bytes, which may exceed a portal's 200KB limit. Above-target outputs are best effort, not proof that a smaller file is impossible.
Is my PDF uploaded to a server?
No. Compression runs inside your browser via WebAssembly, so the file stays on your device and is never uploaded — private by design.
Why can't I select the text after compressing?
'Compress as images' rebuilds every page as a picture, removing text selection and search, including an existing OCR layer. Use the default 'Keep selectable text' method when you need existing selectable text. It does not add text to a scan that has none.
Will compression reduce quality a lot?
It depends on how aggressive your target is. Start with a looser goal (say 200KB), confirm the quality is acceptable, then tighten in steps rather than crushing it to the limit in one go.
Can I compress several PDFs at once?
Yes. Add up to 5 PDFs of 50 MiB each, set a shared target, and choose a method for each file. Each defaults to 'Keep selectable text'. One file's failure does not stop the others; review each result before downloading.
Updated · compress cat team