Documents
PDF Compression
PDF compression reduces the file size of a PDF by re-encoding its images, removing redundant objects and compressing content streams, ideally without visible loss of quality.
Updated 12 Aug 2026
Why PDFs get big
A PDF is a container. Text and vectors are tiny; the weight almost always comes from embedded raster images, embedded font files, and objects left behind by editing — old revisions, unused resources and metadata. A scanned document is really a stack of photographs, which is why scans dwarf text-based exports.
Lossless versus lossy
| Approach | What it does | Quality |
|---|---|---|
| Lossless | Compresses streams, removes duplicate and unused objects | Identical output |
| Image downsampling | Reduces image resolution to a target DPI | Fine for screen, visible when printed |
| Image re-encoding | Re-compresses photos as JPEG at a lower quality | Slight softening, large savings |
| Font subsetting | Keeps only the glyphs actually used | Identical output |
Match resolution to purpose: around 150 DPI is ample for on-screen reading and email, while print usually wants 300 DPI. Downsampling a screen-only document is the single biggest win.
What compression cannot do
Compression does not make a scan searchable — that needs OCR — and it cannot recover detail once images have been downsampled. Always keep the original if the document may later be printed or archived.
Compress PDF
Shrink documents in your browser; the file is never uploaded to a server.
Characteristics
- Operates on images, fonts, streams and unused objects
- Lossless techniques preserve the document exactly
- Lossy techniques trade resolution for size
- Results depend heavily on what the PDF actually contains
Common uses
- Meeting email or upload size limits
- Speeding up web downloads of brochures and reports
- Reducing storage for document archives
- Preparing files for portals with strict size caps
Advantages
- Faster sharing, uploading and downloading
- Lower storage and bandwidth costs
- Lossless passes are risk-free
- Client-side tools keep confidential documents private
Limitations
- Text-only PDFs are already near their minimum size
- Aggressive settings soften images and small type
- Downsampling cannot be reversed
- Compression alone does not make scans searchable
Examples
Text-only report 12 MB -> 11 MB (little to gain)
Photo-heavy brochure 24 MB -> 4 MB (image re-encoding)
300 DPI scan 40 MB -> 6 MB (downsample to 150 DPI)Frequently asked questions
What is PDF compression?
Reducing a PDF's size by re-encoding images, subsetting fonts and stripping redundant objects.
Does compressing a PDF lose quality?
Lossless passes do not. Image downsampling and re-encoding do, though often imperceptibly on screen.
Why is my PDF still large after compressing?
It is probably dominated by high-resolution scans that need downsampling, not just stream compression.
Is it safe to compress confidential PDFs?
Yes, when the tool works in your browser and never uploads the file.
Can I undo compression?
Only by returning to the original file — discarded image detail cannot be restored.
Related terms
Image Compression
Image compression reduces the byte size of a picture by removing redundant or visually unimportant data, either losslessly or by discarding detail the eye barely notices.
OCR
OCR (Optical Character Recognition) converts text inside images and scanned documents into real, selectable and searchable characters.
Minification
Minification removes every byte a machine does not need — whitespace, line breaks, comments and long names — from code or data without changing what it does.
PDF (Portable Document Format) is a file format that preserves the exact layout, fonts and graphics of a document so it renders identically on any device.
Hashing
Hashing converts data of any size into a fixed-length fingerprint using a one-way function, so the same input always produces the same output but the output cannot be reversed.
Character Encoding
Character encoding is the mapping between the characters people read and the bytes computers store, defining how text is turned into binary and back again.
Related tools
Related guides
How to Compress a PDF Without Wrecking the Quality
Understand what actually makes a PDF large, which compression settings matter, how much size you can realistically save, and how to compress a PDF privately in your browser.
How to Compress Images for the Web
A practical guide to image compression: lossy vs lossless, choosing quality levels, resizing before compressing, format choice, and compressing images in your browser.
Compress PDF Without Losing Quality
Honest guide to shrinking a PDF without wrecking it: Lossless (small safe savings), Compress images (re-encode photos, keep text selectable), and Rasterize (biggest savings, flattens pages). Pick the mode that matches your file.
Why Does Compressing an Image Make It Bigger?
When image compression increases file size: re-encoding optimized JPEGs, PNG for photos, quality/format mismatches, and Max quality with no downsampling. How Image Compressor warns instead of faking a win — and what to try next.