How to Compress a PDF

Learn how to reduce PDF file size in your browser, when lossless compression is enough, when to flatten scans, and how to keep text, links and forms intact.

Why PDF files get so large

A PDF is a container. Alongside the text and layout it can hold full-resolution photos, scanned page images, embedded fonts, form fields, color profiles and a history of edits. Most of the weight usually comes from images: a single page scanned at 600 DPI in color can be several megabytes on its own. Other causes are less visible. Some programs write content streams with weak compression or none at all, store each object separately instead of packing them together, or append every edit to the end of the file rather than rewriting it.

Knowing which of these applies to your file tells you which kind of compression will help. A text-based report exported from a word processor usually shrinks with structural cleanup. A 40-page scan from a phone app needs its page images made smaller, and that always involves a trade-off in quality.

Lossless and lossy PDF compression

Lossless compression rewrites the file more efficiently without changing anything you can see or use. The pages look identical, text stays selectable and searchable, links still work and form fields stay fillable. The savings come from packing objects into compressed object streams and recompressing data streams at the highest standard level. How much this helps depends entirely on how the PDF was produced: a carelessly written file can shrink a lot, while a file that is already well optimized may not shrink at all.

Lossy compression throws information away to save space. For PDFs this normally means lowering the resolution or quality of images. The most aggressive form, often used for scans, renders every page as a JPEG image and builds a new PDF from those images. The file gets much smaller, but the pages stop being text: you can no longer search, select or copy words, links stop working and form fields disappear.

How to compress a PDF in your browser

Compress PDF on QuickConvertOnline offers both approaches and runs entirely on your device, using a WebAssembly build of the QPDF library for lossless mode.

1. Open Compress PDF and drop your file onto the page, or select Choose PDF. You can add up to 12 PDFs at once, each up to 50 MB and 500 pages, with 100 MB in total.
2. If the PDF has an open password, type it in the PDF password field and select Open PDF.
3. Under Compression, keep Lossless (preserve text and forms) for documents that must stay searchable, or choose Smaller scans (flatten pages) for scanned or photo-heavy files.
4. For scan mode, pick a resolution between 72 and 300 DPI and adjust the Quality slider. The default is 144 DPI at 85% quality.
5. Select Apply, compare the new size shown in the results, and download the file, or a ZIP if you compressed several PDFs.

When lossless compression is enough

Start with lossless mode every time. It is safe for contracts, forms, manuals and anything else people need to search or fill in, and it costs you nothing in quality. It works best on PDFs produced by older software, by some online form builders and by tools that save edits incrementally.

If the result is not smaller than the original, the tool keeps your original file and tells you the PDF is already optimized. That is a useful signal: the remaining size is real content, usually images, and only a lossy approach or removing pages will make a meaningful difference.

When to flatten scanned pages

Scan mode is designed for documents that are already pictures of paper, such as signed forms photographed with a phone, receipts or archived letters. Because those pages contain no real text, flattening them loses little, while choosing a lower resolution and quality can cut the size dramatically. Before you use it on any other kind of PDF, remember the warning shown in the tool: searchable text, forms and links are removed.

Resolution matters more than quality for file size, because it controls how many pixels each page contains. These starting points work well:

• 72 or 96 DPI: the smallest files, suitable for on-screen previews where fine print does not need to be legible.
• 144 DPI: the default, a good balance for reading on screens and sending by email.
• 200 DPI: sharper small text and better results if the recipient may print the document.
• 300 DPI: print quality, with much larger files. Use it only when the output will be printed.

For black-and-white text scans, a quality between 60% and 75% is usually still easy to read. Zoom in on the smallest text in the result before you send it, and keep your original in case you need a sharper copy later.

Other ways to make a PDF smaller

Compression is not the only option, and sometimes it is not the best one. Consider these alternatives, which keep the remaining pages untouched:

• Remove pages nobody needs, such as blank scan pages, cover sheets or appendices, with Delete PDF pages.
• Send only the relevant section by saving selected pages as a new file with Extract PDF pages.
• Break a very large file into email-sized parts with Split PDF.
• If you are building a PDF from photos, shrink the photos first with the Image compressor and then combine them with Images to PDF.

Passwords, signatures and privacy

Your PDF never leaves your browser during compression, which matters for financial statements, medical records and identity documents. There are two side effects to be aware of. First, when you open a password-protected PDF, the compressed copy is saved without a password, so run Protect PDF afterwards if the file needs encryption. Second, rewriting a PDF that carries a certificate-based digital signature can invalidate that signature. The tool warns you when it detects one, and in that case it is usually better to ask the sender for a smaller original.

Frequently asked questions

How much smaller will my PDF get?

It depends on the file. Lossless mode can shrink inefficiently saved PDFs considerably but may change nothing on an already optimized file. Scan mode at 144 DPI often produces large savings on high-resolution scans.

Will compression make my PDF blurry?

Lossless mode does not change how pages look. Scan mode can soften fine detail, especially at 72 or 96 DPI or at low quality settings, so check the result before sharing it.

Can I compress several PDFs at once?

Yes. Add up to 12 PDFs, apply one setting to all of them and download the results individually or as a single ZIP.