How to reduce PDF file size

Why some PDFs are so large

Text costs very little space in a PDF. A 50-page report with only text and a few charts is often well under 1 MB. What makes PDFs large is nearly always images. A scanner set to 600 DPI in full colour writes about 35 million pixels per A4 page. Photos straight from a phone are often 12 megapixels or more, even when they are printed on the page at the size of a postcard. A brochure exported for a print shop keeps every picture at full print resolution.

Fonts are the second cause. Most programs embed only the letters a document actually uses, which keeps fonts small. But some files embed complete fonts, and fonts for Chinese, Japanese or Korean can be several megabytes each. A third, smaller cause is the file structure itself. A PDF that was edited and saved many times can carry old objects that nothing uses any more, and some programs store parts of the file without compression.

This explains why the same compressor gives very different results. A scan or a PDF of phone photos can shrink to a fraction of its size. A PDF exported from Word with no pictures is already compact, and no tool can make it much smaller without changing how it looks.

Reduce PDF size on a Mac with Preview

Preview, the PDF viewer built into macOS, has a Quartz filter called Reduce File Size. It is quick and needs no extra software, but it is blunt. It downsamples and recompresses every image hard, and the result can look blurry or blocky, especially for scans of small print and for photos. Use it when a file simply has to get under a limit, and zoom in on the result before you send it. Export to a new file name so you keep the original.

1. Open the PDF in Preview.
2. Choose File > Export.
3. In the Quartz Filter menu, choose Reduce File Size.
4. Give the file a new name and click Save, then check the quality of the new file.

Compress a PDF with Ghostscript on the command line

Ghostscript is a free PDF and PostScript interpreter for Linux, macOS and Windows. Its pdfwrite device reads a PDF and writes a new one, and the PDFSETTINGS option picks a preset for image resolution. /screen aims at about 72 DPI and gives the smallest, roughest files. /ebook aims at about 150 DPI and is a good balance for reading on screen. /printer and /prepress keep about 300 DPI for printing.

Because Ghostscript rebuilds the whole file instead of editing it, the output is not always smaller, and some interactive parts of a PDF, such as certain form fields, may not come through the same way. Open the result and compare it with the original. On Windows the program is called gswin64c instead of gs.

# Rebuild the PDF with images at about 150 DPI
gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=small.pdf input.pdf

Compress a PDF losslessly with qpdf

qpdf is a free, open source tool that changes the structure of a PDF without touching what is on the pages. It can compress streams that were stored without compression and pack the many small objects of a PDF into compressed object streams. The pages look exactly the same afterwards. The saving is usually modest, often a few percent and sometimes more for files written by simple generators, and it does nothing for large images. It is a safe first step when the content must not change at all.

qpdf --object-streams=generate --compress-streams=y --recompress-flate input.pdf smaller.pdf

Reduce PDF size with Adobe Acrobat

Adobe Acrobat has a Reduce File Size command, which in recent versions sits with the Compress PDF tool, and the Pro version also has a PDF Optimizer. Reduce File Size works with one click. The Optimizer lets you set the target resolution and compression for colour, greyscale and monochrome images separately, remove embedded fonts you do not need, and discard hidden data such as comments or form data. It also shows an audit of how much space each part of the file uses, which helps when you cannot tell why a file is large. Menu names have moved between Acrobat versions, so search the Tools list for Compress or Optimize if you cannot find them.

Compress a PDF in your browser with FileReshape

FileReshape's Compress PDF tool works in the browser on Windows, Mac, Linux, Chromebooks and phones. The PDF is read and rewritten on your own device and is not uploaded. It offers three methods, and the result always shows the size before and after.

Shrink images, keep text is the recommended method. It finds the JPEG photos and scans in the PDF, scales down those with more pixels than the page needs and saves them again at a lower JPEG quality. You pick Small file (about 96 DPI, JPEG 60 %), Balanced (about 150 DPI, JPEG 72 %) or Better quality (about 200 DPI, JPEG 82 %), measured on an A4 or Letter page. Images that are already smaller are not enlarged, and an image is only replaced when it gets at least 10 % smaller. Text, fonts, links, bookmarks and form fields stay as they are. Only plain RGB or greyscale JPEG images are re-encoded; lossless images such as screenshots, CMYK images in print files, masked images and black-and-white fax-compressed scans are left alone.

Lossless clean-up only is similar to the qpdf command above. It compresses parts stored without compression, writes compressed object streams and drops objects that nothing refers to. Nothing on the pages changes. Pages to images redraws every page as a JPEG at the chosen resolution. That gives the smallest file for scans, but text can no longer be selected or searched, and links, bookmarks and form fields are gone.

If a method would not make the file smaller, you get no download, only both sizes and a suggestion for another method. So you never end up with a bigger file by mistake. The tool handles one PDF at a time. It cannot compress encrypted PDFs, including ones that open without a password but restrict editing, and any change breaks a digital signature, so compress before you sign.

1. Open the Compress PDF page and drop your PDF in the box.
2. Keep the recommended method and the Balanced quality, or pick another.
3. Click Compress PDF, compare the two sizes and download the smaller file.

Remove pages or split the PDF instead

Sometimes the quickest way to a smaller file is to send less of it. If the recipient only needs part of a document, delete the pages they do not need, or extract the few they do. Images that appear only on removed pages are left out of the new file, so a scan shrinks roughly in proportion to the pages you drop.

When an email service refuses a large attachment, splitting can also help. Many services limit an attachment to somewhere between 10 and 25 MB, and some count the encoded size, which is about a third larger than the file itself. Split a 40 MB scan into two or three parts with the split tool and send them in separate emails, or compress the parts as well. If the document is a scan you only need to read on screen, converting the pages to JPG images is another option.

Which method should you use?

Start with the method that changes the least. Try a lossless clean-up when the content must stay exactly the same. For scans and photo-heavy PDFs, shrink the images at about 150 DPI, which is still sharp on a screen and fine for most office printing. Go down to about 96 or 72 DPI only when the file has to fit a strict limit, and check that small print stays readable. Keep the original file in every case, because none of these methods can bring back detail that was thrown away.