⊙ Compress PDF

A large PDF can be awkward to email, slow to upload, and expensive to store. Compressing it reduces the file size while keeping the document readable, so you can share it without hitting attachment limits or waiting on slow transfers.

What compressing a PDF actually does

Most of the weight in a PDF comes from images: scanned pages, photos, and screenshots stored at high resolution. Compression re-encodes those images at a sensible quality and strips redundant data, which can shrink a file dramatically without changing the text or layout.

Documents that are mostly text compress far less, because there is little image data to reduce, but they are usually already small to begin with. The biggest savings come from scans and photo-heavy files, where lowering the stored resolution to something appropriate for screen viewing removes megabytes you never needed. Compression is lossy for images, so the goal is to find the point where the file is small enough while still looking clean.

When compressing a PDF helps

• Your PDF is too big for an email attachment or a web upload form with a size cap.
• You scanned a multi-page document and the resulting file is tens of megabytes.
• You are archiving many PDFs and want to save storage space.
• A portal or government form rejects your submission because the file exceeds its limit.

How Slate compresses PDFs in your browser

Slate opens the PDF locally and re-encodes its pages at the quality level you choose, then rebuilds the file entirely on your device. Nothing is uploaded to a server. You pick a lighter setting to stay crisp or a stronger setting when small size matters most, and Slate shows the before-and-after size so you can confirm the trade-off before you download.

What affects the final size

• The resolution of scanned or photographed pages; higher-resolution scans have the most to gain.
• How many image-heavy pages the document contains versus plain text.
• Whether the file was already compressed once, since a second pass yields little.
• The compression level you pick, which trades visible sharpness against megabytes saved.

Tips for the smallest file at the best quality

• Start with the Recommended level; it balances size and clarity for most documents.
• Use Strong compression for scans you only need to read on screen, and Light compression for anything that will be printed.
• If a file barely shrinks, it is probably already text-based and near its minimum size.
• Compress once at the end of your workflow rather than repeatedly, since each pass can soften images further.