How to reduce PDF file size without losing quality
A 47 MB PDF that should be a 2 MB PDF. You have seen it — a scan of a ten-page document that someone saved at maximum resolution, or a presentation exported with uncompressed images. The file is too large to email, slow to upload, and takes forever to open on a phone. Here is how to shrink it, what each method actually does to the file, and when the size reduction comes at a cost you should know about.
Why PDFs get large in the first place
The text in a PDF takes almost no space. A hundred pages of plain text might be 200 KB. What makes PDFs large is images. A single high-resolution photo embedded in a PDF can be 5-10 MB. A scanned document stores every page as a full image, which is why a ten-page scan can easily reach 50 MB.
Other contributors: embedded fonts (especially if the PDF includes dozens of different fonts), vector graphics with thousands of anchor points, and layers or annotations from editing software. But in practice, images are the issue ninety percent of the time.
Method 1: Re-export from the source
If you created the PDF from Word, PowerPoint, InDesign, or any other application, the most effective approach is to go back to the source and export again with lower image quality settings.
In Microsoft Word, go to File, Save As, PDF, then click Options or More Options. Look for an "Optimize for" setting — "Minimum size" produces a significantly smaller file by downsampling images. In PowerPoint, the same option exists during PDF export.
In Adobe InDesign, the PDF export dialog has detailed compression settings for each image type. Setting color images to JPEG compression at Medium quality and downsampling to 150 DPI will dramatically reduce file size while keeping the output perfectly readable on screen and in print.
This method produces the best results because the application has access to the original image data and can resample it intelligently. If you have the source file, always start here.
Method 2: macOS Preview (Mac only)
On a Mac, open the PDF in Preview. Go to File, Export. In the "Quartz Filter" dropdown, select "Reduce File Size." Save. Done.
The catch: Apple's default "Reduce File Size" filter is aggressive. It downsamples images to 512 pixels on the longest side, which makes photos look noticeably blurry. For text-heavy documents with small diagrams, the result is usually fine. For documents with photos or detailed graphics, the quality loss may be unacceptable.
You can create custom Quartz filters with better compression settings using the ColorSync Utility, but that requires some technical setup. For a quick reduction where image quality is not critical, the built-in filter works.
Method 3: Ghostscript (Windows, Mac, Linux)
Ghostscript is a free, open-source tool that processes PDF files from the command line. It is the engine behind many PDF compression tools and gives you fine control over the output.
The basic command for compression looks like this:
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dBATCH -sOutputFile=output.pdf input.pdf
The -dPDFSETTINGS flag controls the quality level. /screen is the smallest (72 DPI images, suitable for on-screen viewing only). /ebook is a good middle ground (150 DPI). /printer preserves more quality (300 DPI). /prepress keeps nearly everything at original quality.
For most documents, /ebook reduces file size by 60-80% while keeping text sharp and images clear enough for any screen. The result is too low-resolution for high-quality printing, but that is rarely the use case for emailing a PDF.
Method 4: Online compression tools
Services like Smallpdf, ILovePDF, and Adobe Acrobat Online offer one-click PDF compression. Upload your file, choose a quality level, download the result. They work well and the results are usually good.
The same privacy consideration applies as with any online file tool: your PDF goes to someone else's server. If the document contains anything sensitive, this is a risk. For a product brochure or a public report, it is fine. For a financial statement or a contract, use a local method instead.
Method 5: Print to PDF at lower quality
A blunt but effective approach: open the PDF and "print" it to a new PDF file using your operating system's virtual PDF printer. On Windows, select "Microsoft Print to PDF" as the printer. On Mac, use the PDF button in the print dialog.
This re-renders the document and often strips out hidden data, unused fonts, and uncompressed image streams. The resulting file is frequently 30-50% smaller with no visible quality difference. It also strips metadata, which may or may not be what you want — if you need a clean file, this is a two-for-one.
What you lose when you compress
Every compression method reduces image resolution, re-encodes images with lossy compression, or both. Text stays sharp because it is stored as vectors, not pixels. But images get downsampled, and if the PDF contains high-resolution photos that the recipient needs at full quality — for printing, for design work, for medical imaging — compression will degrade them.
For documents that will be read on screen, 150 DPI is more than enough. For documents that will be printed at high quality, keep the resolution above 300 DPI. For anything destined for a phone screen or email attachment, even 100 DPI looks fine.
The rule of thumb: compress a copy, never the original. Keep the full-size version and send the compressed one.