Compress PDF online — Free & Private
Reduce PDF file size while keeping text fully editable and selectable. Processed entirely in your browser — no uploads, no account, no watermark.
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How to compress a PDF file online
Compressing a PDF with PDFluna takes less than a minute and happens entirely in your browser. There is nothing to install, no account to create, and no files are sent to a remote server — your document is processed locally using JavaScript and Web Workers. Here is the full process, step by step:
- Upload a PDF. Drop your file onto the upload area or click Choose PDF file. The tool accepts any standard PDF up to 100 MB.
- Pick a compression level. Use Low if you plan to print the document, Medium for everyday sharing over email and chat, or High when you need the smallest possible PDF file size.
- Compress in your browser. PDFluna re-encodes embedded images, strips unused metadata and rewrites the PDF with compact object streams — all on your device.
- Download the smaller PDF. When processing finishes you’ll see the before/after size and can save the compressed file. The original is never modified.
Why compress PDF files?
PDFs are everywhere — contracts, invoices, scanned documents, portfolios, e-books — and they are easy to create but often very large. A single scanned page at 300 DPI can weigh several megabytes, and a report full of screenshots will quickly blow past email attachment limits. A good PDF compressor makes your documents practical to share and fast to load, without changing the format.
PDFluna’s online PDF compressor is optimised for three common jobs: sending files as email attachments (most mail providers cap attachments around 25 MB), uploading documents to web forms that reject heavy files, and keeping archives small. If you also need to merge multiple PDFs, split a document into parts, rotate sideways scan pages or edit your PDF, PDFluna has free browser-based tools for those too.
How PDFluna compresses PDFs
Under the hood, PDFluna uses two open-source libraries: Mozilla’s PDF.js to parse pages and locate embedded images, and pdf-lib to rebuild the file. Heavy work runs inside a Web Worker so the page never freezes. The compressor focuses on four areas where PDF files typically waste space:
- Image re-encoding. Embedded photos and scanned pages are re-drawn onto an
OffscreenCanvasat a lower DPI and exported as JPEG at a quality tuned to the chosen level. - Metadata cleanup. XMP metadata, creator strings and producer tags are cleared because they have no visual meaning.
- Object streams. The PDF is saved with
useObjectStreams: true, which packs multiple small objects into a single compressed stream. - Deflate compression. Content streams are compressed with DEFLATE, the same algorithm used by zip and gzip.
Is it safe to compress PDF online?
With PDFluna, yes — because nothing is actually "online" about the compression step. Once the page has loaded in your browser, the PDF is read into memory using the standard File API and processed with JavaScript inside a Web Worker on your own machine. We never upload it, never see it and never store it. That makes PDFluna a safe choice for sensitive documents such as contracts, tax returns or medical forms. If you ever need a second check, you can open browser DevTools and watch the Network panel — no requests are made during compression.
Frequently asked questions
How much can I compress a PDF?
Typical image-heavy PDFs shrink by 40–80% on the High level. Text-only files compress less (around 5–15%) because there are no images to re-encode. If the original is already well optimised, PDFluna will keep it instead of producing a larger file.
Is PDFluna safe to use?
Yes. All compression happens in your browser using JavaScript and Web Workers. Your PDF is never uploaded to any server — it never leaves your device. You can confirm this yourself in your browser’s Network panel.
Does compression reduce quality?
Low keeps images at print-ready quality and is barely distinguishable from the original. Medium is balanced for screen viewing. High is optimised for email and web, so you may notice slightly softer images on heavily illustrated pages.
What is the maximum file size?
The tool accepts PDFs up to 100 MB. Compression is the most memory-intensive thing a PDF tool does — every embedded image is decoded, re-encoded and the whole file is rebuilt — so very large files (around 60 MB and up) are best run on a computer rather than a phone, where the browser tab can run low on memory. You’ll see a heads-up note when a file is large enough to matter.
Do I need to register or install anything?
No. PDFluna Compress PDF is a free web tool. There is no sign-up, no software to install and no watermark added to the compressed file.
Why is my compressed PDF larger than the original?
Some PDFs are already well optimised — for example, files that were exported from modern word processors or previously compressed. When re-compression would actually increase the size, PDFluna keeps your original so you never get a worse version.
Can I compress a PDF to a specific size, like 100 KB or 1 MB?
There’s no exact-size dial — results depend on what’s inside the file — but the targets are predictable: 1 MB is reachable for almost any document on the High level, 200 KB works for text files and short scans, and 100 KB fits 1–2 pages of text or a single grayscale scan page. For the realistic numbers per file type and what to do when the target is physically impossible, see our guide on compressing a PDF to 100 KB, 200 KB or 1 MB.