The Complete Guide to PDF Compression
PDF compression shrinks a PDF's file size by re-encoding its pages and down-sampling the images inside it. 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 as readable as possible. This guide explains how compression works, the difference between lossy and lossless, when you actually need it, how to hit an exact target size with compress cat, and how to troubleshoot a file that won't shrink. compress cat runs entirely in your browser — your files are never uploaded.
What is PDF compression?
PDF compression makes a PDF smaller without destroying its readability. Most of a PDF's size comes from the images on its pages — scans, ID photos and phone snapshots of documents are all bitmaps, and the higher their resolution and color depth, the bigger the file. Compression re-encodes those images: it lowers their resolution (DPI), raises the JPEG compression ratio, and drops color data you don't need.
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 — duplicate objects, unused fonts, metadata, thumbnail caches — without discarding a single pixel. Quality is untouched, but the savings are modest (usually 10–30%), nowhere near enough to hit a hard target like 200KB.
Lossy compression actually throws away some image detail — lower resolution, higher JPEG quantization — in exchange for a much smaller file. The trade-off is slightly softer images and, occasionally, faintly fuzzy text edges. Almost every 'compress to an exact size' task needs lossy compression, because only it can take a multi-megabyte scan down to a few hundred KB.
- Lossless: quality unchanged, low ratio (~10–30%) — good for files that are already smallish.
- Lossy: some quality loss, high ratio — the only realistic way to hit '200KB / 500KB / 1MB' limits.
- Practical tip: set the target size first and let the tool balance clarity against size, instead of guessing settings by hand.
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 compress a PDF to an exact size?
compress cat is built for beating upload limits: you tell it the target size, and it binary-searches between clarity and size to land just under your goal — no manual DPI or quality tweaking required.
- Open the Compress PDF tool and drag your PDF in, or click to choose files (you can add several at once).
- Tick 'Compress to a target size' and enter your goal, or click a 100KB / 200KB / 500KB / 1MB preset.
- Click Start — compression runs locally in your browser and converges on your target.
- Check the before/after for each file and download. If a file's content is too dense to hit the target, you get its smallest possible result with a 'best effort' note.
Why won't my PDF compress? Troubleshooting
'It won't get smaller' is usually a sign the file is already near the floor of its information density, not a tool problem. Here are the common cases and how to handle them.
- The images are already heavily compressed: squeezing further only degrades quality. Loosen the target a little.
- Too many pages: a 100-page scan won't fit in 200KB. Split the PDF or delete blank/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.
- Losing selectable text after compression is expected: compress cat rasterizes pages, which is ideal for scans — see the next section.
Scanned vs. text PDFs: why it matters
This is the single biggest factor in how well a PDF compresses. A scan (including a phone photo turned into a PDF) is essentially one image per page, so down-sampling can easily cut 80–95% of the size without breaking anything — there was never selectable text to lose.
In a text PDF (exported from Word, Google Docs and the like) the text is vector data and barely adds to the size. Because compress cat compresses by rasterizing — turning each page into an image — the text stays visible but can no longer be selected, copied or searched. If you need the text to stay selectable, don't rasterize a pure-text document; those files are usually small enough that they don't need compressing anyway.
- Scans / photo PDFs: high ratio, great results — compress cat's ideal input.
- Pure-text PDFs: text becomes an image after compression; usually small already, so often no need to compress.
- Mixed PDFs (text + big images): compress the source images first, or accept losing selectable text after rasterizing.
Frequently asked questions
Can a PDF be compressed to exactly 200KB?
Yes. compress cat binary-searches between clarity and size to converge on the target you set (100KB / 200KB / 500KB / 1MB). Scans and image-based PDFs usually land under the target reliably; if a file is too dense, you get its smallest possible result with a 'best effort' note.
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 cat rasterizes pages, which is best for scans and photo PDFs. A pure-text PDF becomes an image after compression, so text can no longer be selected or searched. If you need selectable text, don't compress a pure-text document with this tool.
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. compress cat lets you select multiple PDFs and compress them all to the same target. If you also need to merge or watermark them afterwards, chain the steps into a reusable pipeline with the workflow builder.
Updated · compress cat team