Compress a PDF (reduce file size)

To compress a PDF, re-save it with lower image resolution using Acrobat's Reduce File Size, Preview's Reduce File Size filter on Mac, or Ghostscript on any system. PDF Viewer on this site does not compress PDFs; it opens, annotates and OCRs them in the browser without uploading. This guide explains what makes a PDF big and which method shrinks it without ruining the pages.

What makes a PDF big

Text is cheap: a hundred pages of plain text weigh under a megabyte. The size comes from three things.

• Scanned pages and photos. A 300 DPI color scan is 2 to 5 MB per page before compression. Scans are the usual reason a PDF is 50 MB.
• Embedded fonts. A full CJK font can be 10 MB; most exporters embed only the glyphs used, but some do not.
• Leftovers. Deleted pages still present in the file, duplicate images, attached files, high-resolution thumbnails, and uncompressed object streams from older generators.

Open the file in the viewer and check Document properties for the producer and page count; a 20-page file at 40 MB is almost certainly images. If the file is a scan you also want to search, run OCR after compressing, not before, so the text layer is built from the final images.

Which compress PDF method for which case

Main method: Ghostscript, with Acrobat and Preview alternatives

Ghostscript (free, any OS):

1. Install Ghostscript (gs on Mac and Linux via Homebrew or the package manager, gswin64c on Windows).
2. Run gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -dNOPAUSE -dBATCH -sOutputFile=small.pdf input.pdf.
3. Presets: /screen (72 DPI images, smallest), /ebook (150 DPI, good for sharing), /printer (300 DPI), /prepress (keeps nearly everything).
4. Compare sizes; if /ebook is still too large, the file is mostly scans and /screen is the next step.

Acrobat Pro: File, Save As Other, Reduced Size PDF, choose compatibility with recent Acrobat versions, Save. For finer control use the PDF Optimizer (Advanced Optimization) to downsample images and unembed fonts.

Preview (Mac): File, Export, keep Format PDF, set Quartz Filter to Reduce File Size, Save under a new name. The built-in filter downsamples hard; for a gentler version, create a custom filter in ColorSync Utility with 150 DPI images.

Online compressors are the simplest route on a phone. They upload the file, so keep contracts, IDs and medical records to the local methods.

What gets lost when you compress a PDF

• Image detail. Downsampling to 150 DPI is invisible on screen but visible in print; 72 DPI blurs small text in scans.
• Color. Aggressive presets convert to sRGB and may flatten transparency.
• Fonts. Unembedding saves space but the PDF then depends on the reader's installed fonts.
• Interactive parts. Some tools drop form fields, bookmarks, links or annotations; Ghostscript's pdfwrite keeps most, Preview's filter is less predictable.
• Text layer in scans. Usually kept, but verify with Ctrl+F.

A compressed scan can still be searchable; see how to make a scanned PDF searchable if the text layer is missing afterward.

Prevent the big file at the source

Compressing after the fact always costs something. Most oversized PDFs can be avoided one step earlier.

• Scanner settings. Scan text documents at 200 DPI in grayscale or black and white, not 600 DPI color. A page drops from 5 MB to under 300 KB with no visible change on screen.
• Phone scans. Scanner apps and the Notes document scanner produce smaller, straightened pages than the camera roll; choose the document or grayscale mode.
• Word, PowerPoint, Excel. Compress Pictures before saving, pick Minimum size in the PDF options, and do not paste screenshots at full resolution when a crop will do.
• Design tools. In InDesign or Affinity, use a print-ready or web preset instead of the default that embeds full-resolution images.
• Repeated edits. Each round of adding annotations and re-saving in some tools appends to the file instead of rewriting it. A final Save As or a Ghostscript pass rewrites it cleanly.

Done this way, the PDF rarely needs compressing at all, and the copy you send is the copy you checked.

Check the result

Open both files in the online PDF viewer, one per tab. Same page count, pages zoomed to 200 percent still readable, Ctrl+F finds the same words, annotations and links still present. If the compressed file fails any of these, go one preset up and try again. Nothing is uploaded during the check.

Frequently asked questions