Does compressing a PDF reduce image quality?

Not with PDFly's compressor. It rebuilds the PDF's internal structure losslessly and does not re-encode or downsample embedded images, so there is no visible quality loss. For pixel-level image compression, use PDFly's Image Compressor on the images themselves.

🗜️ PDF Compressor

Compress PDF

Reduce PDF file size without losing quality. Choose your compression level and download instantly.

Last Updated: August 2026Written by Ayaz

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Compress your PDF

Note: This tool shrinks a PDF's internal structure, not its embedded images — so form-heavy or frequently-edited PDFs shrink the most, while scanned or photo-heavy PDFs may see little change. See "Why some PDFs barely shrink" below. All compression happens locally — files are never uploaded.

How to compress a PDF online

Drop your PDF into the tool above, or click to browse your device.
Pick a compression level — Low, Medium, or High.
Click "Compress PDF" and download the result. Nothing is uploaded; the file is compressed inside your browser.

How PDFly compresses your PDF

A PDF's file size comes from two very different places: the document's internal structure (its object table, fonts, form fields, and metadata) and the images embedded inside it. Shrinking either one counts as "compression," but the techniques are unrelated.

PDFly's Compress PDF tool runs entirely in your browser using the pdf-lib library. When you compress a file, it rebuilds the PDF's internal object table, removes duplicate objects and leftover data from previous edits, and — on Medium and High — rewrites that table using compressed object streams, a more compact format introduced in PDF 1.5. This whole process is lossless: it doesn't touch a single pixel in your images or re-encode a single embedded font, so there's no quality to lose.

In plain terms: PDFly's compressor does not currently re-encode, downsample, or recompress embedded images — the technique that does the most for scanned pages and photo-heavy PDFs. For that, use PDFly's Image Compressor on the images themselves, or the workflow described below.

Lossless vs. lossy compression

Lossless compression reorganizes data more efficiently without discarding anything — what PDFly's tool does. Lossy compression discards detail to save more space, typically by re-encoding a JPEG at a lower quality setting or reducing its pixel dimensions (downsampling) — something desktop PDF editors like Acrobat can do, but which PDFly's in-browser engine doesn't perform today. If a PDF's size is dominated by its images, lossless structural compression alone will only get you so far.

Choosing a compression level

Low: skips the object-stream rebuild, keeping the older, more universally compatible cross-reference format — useful if the file needs to open in a very old PDF reader.
Medium: the recommended default. Rebuilds the file with compressed object streams for a smaller, still fully compatible PDF.
High: currently applies the same structural rebuild as Medium. It's PDFly's strongest available setting today, and the level we'll extend first as image-aware compression is added to the engine.

Why some PDFs barely shrink

Scanned pages: the file is almost entirely embedded page images, which structural compression doesn't touch.
Already-optimized PDFs: files previously saved with "optimize" or "reduce size" in another tool have little redundant structure left to remove.
Encrypted or password-protected PDFs: the object table can't always be rebuilt cleanly — see Troubleshooting below.
Mostly-text or mostly-vector PDFs: these are usually small to begin with, so there's less to gain in absolute terms even when the percentage looks decent.

Scanned documents and photo-heavy PDFs

If a scanned PDF needs to be smaller, the real lever is the images, not the wrapper around them. Two workflows actually move the needle: scan at a lower resolution in the first place (150–200 DPI is plenty for on-screen reading; save 300 DPI for documents that will be printed), or, for a PDF you already have, pull the pages out with Extract Images, shrink them with Image Compressor, and rebuild the PDF with JPG to PDF. If you also need the text searchable, run OCR PDF before or after.

Before you compress: quick wins

Remove Pages: drop pages you don't need with Remove Pages before compressing — fewer pages is a bigger win than any compression setting.
Organize PDF: reorder or delete pages in one pass with Organize PDF.
Flatten PDF: on heavily annotated or form-filled documents, Flatten PDF can reduce stored object overhead.
Merge once: if you're combining files with Merge PDF, compress the sources first rather than merging repeatedly and compressing at the end.

Common mistakes

Expecting a scanned or photo-heavy PDF to shrink dramatically from structural compression alone.
Compressing before trimming pages you don't need — compress last, once the file only contains what you're keeping.
Running the compressor repeatedly on the same file hoping for extra savings each pass — once the object table is rebuilt, there's nothing left to rebuild.
Deleting the original before checking that the compressed file opens correctly, especially for PDFs with forms or complex layouts.

Troubleshooting

Password-protected PDF: remove the password first with Unlock PDF. Compressing an encrypted file directly can produce a corrupted result.
File won't open after compressing: try Repair PDF on the original first, then compress the repaired version.
Result says "Optimized" instead of a percentage: that means the reduction was under 1% — normal for a file that was already efficiently structured.
Large files are slow or the tab freezes: everything happens in your device's memory, so files of several hundred MB can be slow on phones or older computers. A desktop browser handles large PDFs more comfortably.

About compress PDF

Reduce PDF file size for easier sharing, storage, and uploading. PDFly offers compression choices so you can balance smaller files with the visual quality you need.

Common uses for compress PDF

Helpful for email attachments, online application limits, and document archives. Works best on form-heavy or frequently-edited PDFs, since it rebuilds the file's internal structure — scanned or photo-heavy PDFs shrink less here, because their size comes mostly from embedded images, which this tool doesn't re-encode (use Image Compressor for that).

Tips for using this PDF tool

Keep the original file when quality matters. For scans, check a few pages after compression to make sure small text, signatures, and diagrams remain readable.

Related PDF tools

Want a step-by-step walkthrough? Read the Compress PDF guide.

Frequently asked questions

Not with PDFly's compressor. It rebuilds the PDF's internal structure losslessly and doesn't re-encode or downsample embedded images, so there's no visible quality loss. For pixel-level image compression, use Image Compressor on the images themselves.

Scanned PDFs are almost entirely made up of embedded page images. Structural compression only rebuilds the document's object table, so it can't shrink the images that make up most of a scanned file's size.

Low skips the object-stream rebuild for maximum compatibility with older PDF readers. Medium rebuilds the file using compressed object streams and is the recommended default. High currently applies the same structural rebuild as Medium and is reserved for stronger, image-aware compression as PDFly's engine develops.

If the size reduction is under 1%, PDFly shows "Optimized" instead of a misleadingly small or rounded-to-zero percentage. This is common on PDFs that were already efficiently structured before compression.

No. PDFly's PDF Compressor runs entirely in your browser using the pdf-lib library. Your file is read, compressed, and made available for download without ever being sent anywhere.

Remove the password first with Unlock PDF. Compressing an encrypted file directly can produce a corrupted or unreadable result.

No. The compression process rewrites the file's internal structure without altering its content, so form fields, hyperlinks, and bookmarks are preserved.

There's no hard limit, but since compression runs in your browser's memory, very large files (several hundred MB) can be slow on phones or older computers. A desktop browser will handle large PDFs more comfortably.

Common workflow

Merge PDF FilesCompress PDFProtect PDF

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