Compress PDF

What is PDF compression?

PDF compression reduces the file size of PDF documents by optimizing images, removing unnecessary metadata, and streamlining the document structure. Large PDFs are common when documents contain high-resolution images, embedded fonts, or redundant data. Compression makes files easier to email, faster to upload, and cheaper to store.

This tool compresses PDFs in your browser — no server upload required. Your documents remain completely private. The compression focuses primarily on image optimization (which accounts for the majority of PDF file size) and removal of unnecessary metadata while preserving text quality and document structure.

Why do PDFs get so large?

The single biggest reason a PDF grows large is high-resolution images. Every photograph, scan, or diagram embedded in a PDF stores pixel data. A page scanned at 600 DPI produces roughly four times as many pixels as the same page scanned at 300 DPI — and eight times as many as 150 DPI — so the file size grows quickly. A multi-page document scanned at 600 DPI can easily reach 50–100 MB before any compression.

Embedded fonts are the second contributor. PDF files can embed the full font file so the document looks identical on any device, even if the recipient does not have that font installed. A single font can add 200–500 KB; a document using several custom typefaces starts to add up. The PDF specification supports font subsetting, where only the specific characters that appear in the document are embedded — this can cut font overhead by 60–80%.

Other space consumers include document history (revision data left over from editing software), hidden layers, thumbnail previews, duplicate embedded objects, and XML metadata. These are usually small individually but can collectively add several megabytes to a complex document.

How PDF compression works

The primary technique is image downsampling. Each embedded image is rescaled to a lower pixel density before re-compressing. For a document intended to be read on screen, 150 DPI is generally indistinguishable from 300 DPI at normal viewing distances. A 600 DPI scanned image downsampled to 150 DPI reduces that image's file size to roughly 1/16th of the original. When a PDF contains many scanned pages, this single step delivers the dramatic file-size drops you see with high-compression settings.

After downsampling, images are re-encoded with JPEG compression. JPEG uses a quality factor — roughly 85% quality retains nearly all visible detail while cutting file size significantly; dropping to 60% achieves aggressive savings with noticeable softening only in close-up inspection of photographs. Text-based scans are more sensitive: even mild JPEG compression can make small printed text look slightly blurry. For documents where crisp text in scanned pages matters, use the Low compression setting.

Beyond images, compression also strips unused metadata, removes duplicate embedded resources (two pages using the same logo only need to store it once), and applies lossless compression to the PDF structure itself using the DEFLATE algorithm. These lossless steps never degrade quality — they are always safe to apply.

What gets compressed

Images are downsampled to a lower resolution and re-compressed — this produces the biggest size savings since images often account for 80-95% of a PDF's file size. Embedded fonts may be subsetted (only the characters used in the document are kept). Duplicate objects are merged. Unused metadata and document history are stripped. Text and vector graphics remain unchanged.

Choosing between quality and size

There is no single right answer — the best compression level depends on how the document will be used. A PDF of a printed brochure that a commercial printer will produce from should stay at Low: print requires 300 DPI images and any quality loss will be visible in the final product. A PDF you are attaching to an email or uploading to a web form can comfortably use Medium: the recipient will view it on a screen at normal zoom, and 150 DPI looks identical to 600 DPI on a standard monitor.

High compression is best reserved for documents where size is the only priority — for instance, archiving thousands of scanned pages where storage cost matters, or sharing a large report through a messaging app with a strict attachment limit. At High, images are compressed aggressively and photographs will appear visibly softer at 100% zoom. Text in a well-formatted (non-scanned) PDF is unaffected because it is stored as vectors, not pixels.

Compression levels

• Low — minimal quality loss, 10-30% size reduction. Best for documents you will print.
• Medium — good balance of quality and size, 30-60% reduction. Suitable for most uses.
• High — maximum compression, 50-80% reduction. Best for email and web sharing where file size matters most.

Common use cases for PDF compression

Email is the most frequent reason people need to shrink a PDF. Gmail, Outlook, and most webmail services cap attachments at 25 MB. Sending a 40-page scanned report can easily exceed this limit, and the sender gets a bounce without a clear error message. A single compression pass typically brings such a file well within limits.

• Email attachments — stay under the 25 MB Gmail / Outlook limit
• Web upload forms — many government, HR, and legal portals cap uploads at 5–10 MB
• Cloud storage and sharing — smaller files sync faster and consume less storage quota
• Embedding PDFs in websites — smaller files load faster for site visitors
• Long-term archiving — compressing thousands of scanned documents saves significant storage cost over time

Tips for better results

If you control the scanning process, scan text-only documents at 200–300 DPI rather than 600 DPI. The resulting PDF will be much smaller before compression, and character recognition (OCR) software works just as well at 300 DPI as at 600 DPI. Scanning in black-and-white (1-bit or grayscale) rather than full colour also cuts file size dramatically — a colour scan at 300 DPI can be five to ten times larger than a grayscale scan of the same page.

For documents you create in word processors or presentation software, export directly to PDF rather than printing to PDF through a virtual printer driver. Direct export produces a clean, properly structured PDF with subsetted fonts and compressed images. The "Print to PDF" route sometimes embeds full unsubsetted fonts and high-resolution images unnecessarily.

Common mistakes to avoid

• Over-compressing scanned text — applying High compression to a PDF of scanned typed text can make small characters look blurry and hard to read. Use Low or Medium for any document where legibility of small print matters.
• Expecting large savings on text-only PDFs — a PDF created directly from a word processor with no embedded images typically compresses by only 5–15%. Most of its content is already efficient vector text; there are no large images to downsample.
• Re-compressing an already-compressed PDF — running the same PDF through a compressor multiple times rarely achieves additional savings after the first pass. JPEG images that have already been compressed do not compress further; they only degrade.
• Compressing a file you need to edit later — compression strips embedded revision data and may downsample images to a resolution that is too low for re-editing. Always keep the original high-resolution file and compress a copy for sharing.

How to use this tool

Upload a PDF and select a compression level (low, medium, or high). Low compression preserves the best quality with modest size reduction. High compression achieves the most size reduction but images may appear slightly softer. The tool shows the original and compressed file sizes with the percentage reduction.

Frequently asked questions

Will compression affect text quality?

No. Text in PDFs is stored as vector data (mathematical descriptions of each character), not as images. Compression does not alter text — it remains sharp at any zoom level. Only raster images (photographs, scanned pages) are affected by compression. If your PDF is entirely text-based, compression may achieve only modest size reduction.

How much can a PDF be compressed?

It depends on the content. PDFs full of high-resolution photos (like photo albums or scanned documents) can often be reduced by 70-90%. Text-heavy PDFs with few images may only be reduced by 5-20%. Already-compressed PDFs (from a previous compression tool) will show minimal further reduction. The tool shows the exact savings after processing.

Is it safe to compress a PDF that contains sensitive information?

With this tool, yes — the compression runs entirely inside your browser using JavaScript. Your PDF is never uploaded to any server and never leaves your device. Once the page is loaded, you can even disconnect from the internet and the tool will still work. No one else can access your document.

What if my PDF is already small?

If your PDF is already under 1 MB there is usually little benefit to compressing it further. Text-only PDFs and PDFs that have already been compressed are largely incompressible. The tool will still process the file and report the output size, but do not be surprised if the saving is under 10%.