How Do You Compress a PDF? The Complete 2026 Guide for Every Platform

To compress a PDF, use a free online tool like Adobe’s online compressor or Preview on Mac. Both reduce file size dramatically without installation. For Windows, a local desktop app or the “Save as PDF” trick works offline.

Why Your PDF Is So Large: Image Resolution and Embedded Data

Before you compress a PDF, it helps to understand what makes it large. High-resolution images and excessive DPI are the number one cause of oversized PDFs. A single full-page photograph at 300 DPI can add 5–10 MB. If your document contains scanned contracts, brochures, or photo-heavy reports, image data can account for 80–90% of the total file size.

Embedded JPEGs and uncompressed content streams also bloat the file. When a PDF is created, it may store images at very high quality and keep the content stream in an uncompressed state. Hidden metadata, annotations, embedded fonts, and version history further increase the file size without adding visible value.

How DPI Affects PDF File Size

DPI, or dots per inch, determines how many pixels are stored per inch of an image. Using the right DPI setting is critical because unnecessary pixels are the primary source of bloat. According to Technology Org, a table of typical DPI settings clarifies the trade-off: the /screen setting uses 72 DPI, /ebook uses 150 DPI, and /printer uses 300 DPI. For screen viewing and email attachments, /ebook at 150 DPI is often sufficient; it can reduce file size by 50–70% with no visible quality loss in most text-based documents. For text-only PDFs, compressors can achieve a 70% size reduction with zero visible loss, as noted by Free PDF Compress.

How to Compress a PDF: 4 Quick Methods for Any Platform

Here are four reliable methods for compressing a PDF on Windows, Mac, or online. Each method includes the expected file size reduction, the tool you need, and the quality trade-offs.

Compress Online in Seconds (No Installation)

The fastest way to compress a PDF is with a free online tool. Adobe offers a free online compressor at its website. Go to the site, drag and drop your file, select one of three compression levels (High, Medium, or Low), and click Compress. High compression produces the smallest file suitable for email attachments; Medium balances size and clarity; Low maintains the highest quality. The tool works in seconds, but it requires an internet connection and cannot process password-protected files.

Other reliable online services include Smallpdf, iLovePDF, and PDF24. The process is nearly identical: upload your PDF, select a compression level, and download the result. These services generally have size limits (~100 MB) and may restrict the number of free daily uses.

Compress on Mac with the Built-in Preview App

Mac users have a built-in tool called Preview. Open your PDF in Preview, click File → Export, then check “Optimize image for screen” in the dialog box. Click Save. This method re-encodes images at a lower resolution. For scanned or photo-heavy documents, the reduction can be substantial: a 20-page scanned research paper at 45 MB might drop to 8–12 MB. Be aware that this is a lossy process, so keep a backup if you need the original quality. On macOS Sonoma (14) or older, you will see a “Quartz Filter” dropdown; select “Reduce File Size” instead.

Compress on Windows Using Adobe Acrobat Pro

Adobe Acrobat Pro (paid) provides granular control. Open the Optimize PDF tool from the Tools tab, select your file, click Reduce File Size, and choose a compatibility option. For more advanced control, go to File → Save as Other → Optimized PDF. In the PDF Optimizer, you can downsample images to a specific DPI, select compression algorithms (JPEG, JPEG2000, ZIP), subset fonts, strip metadata, and flatten annotations. The “Audit Space Usage” button shows exactly what consumes space—images, fonts, or content streams. Acrobat Pro is best for users who need consistent quality across many files.

The ‘Print to PDF’ Method and Why It Can Backfire

A common workaround on Windows is to use the “Print to PDF” dialog, but this can backfire. According to PDF Candy, a file created using “Print to PDF” resulted in a 17 MB file, while the exact same original document converted using “Save as PDF” in Microsoft Office produced a file of only 0.5 MB. The issue is that “Print to PDF” often converts text and images into large, non-editable content streams that resist later compression. This method is unreliable and should be avoided. Instead, always use the “Save as PDF” option from within the source application (e.g., Word, Excel).

Which PDF Compression Method Should You Choose? A Decision Flow

Choosing the right method depends on your skill level, privacy needs, platform, file count, and budget. Use this simple decision tree:

• Need a quick, single file with low privacy concerns? → Use an online tool (Adobe Online Compressor, Smallpdf).
• On a Mac, offline only? → Use Preview’s export feature.
• On Windows, need offline control? → Use Adobe Acrobat Pro or PDF Candy Desktop.
• Need batch compression or automation? → Use command-line Ghostscript.
• Privacy is critical (confidential docs)? → Use a desktop app that processes locally (Preview, Adobe Acrobat Pro).

Quick Reference: When to Use Online vs Offline

Advanced Techniques: Compressing Stubborn PDFs with Content Streams and Font Subsetting

Some PDFs are stubborn—they refuse to shrink even after compression. This is often due to content streams and fully embedded fonts. Advanced techniques can handle these cases.

What Are Content Streams and Why Do They Matter?

Content streams are the raw data that describe the entire layout of a PDF page, including text and line drawings. As explained by Neuxpower, when a document is converted via “Print to PDF,” text and images are stored in large, opaque content streams that typical compressors cannot manipulate. The only reliable solution is to go back to the source document and use “Save as PDF” instead. If you cannot access the source, a workaround is to re-print the PDF through a web browser’s printer and select “Save as PDF” as the destination. This sometimes creates a new PDF with more manageable content streams.

Font Subsetting: A Free Way to Shed Extra KB

Font subsetting embeds only the characters actually used in the document, rather than the entire font family. A single embedded font family can add 400–600 KB. Most modern PDF editors, including Adobe Acrobat Pro and Ghostscript, support font subsetting. In Acrobat, use the PDF Optimizer to enable subsetting. This technique can reduce file size by 5–15% with no impact on text quality.

Using Ghostscript for Batch Compression (Advanced)

Ghostscript is a free, open-source command-line tool that runs on Windows, Mac, and Linux. A basic compression command looks like this:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dBATCH -sOutputFile=compressed.pdf input.pdf

The -dPDFSETTINGS flag controls quality: /screen for maximum compression (72 DPI), /ebook for a good middle ground (150 DPI), /printer for print quality (300 DPI). Ghostscript is ideal for automating batch compression. For example, scanned color PDFs can be reduced by 91% with minimal quality loss, as noted by Free PDF Compress.

How to Compress a PDF on Mobile (Android & iOS)

Compress PDFs on Android

The PDF Compressor: Resizer & Zip app from SoulApps (updated April 28, 2026) is a free, offline tool. Open the app, select a PDF, choose a compression level (Best Quality, Balanced, or Smallest Size), and compress. The app works 100% offline, so your files stay on your device. It also includes tools for merging, splitting, and converting PDFs. No sign-in or watermark is required.

Compress PDFs on iPhone and iPad

On iOS, the Adobe Fill & Sign app includes a PDF compression feature. Alternatively, you can use the built-in Files app with third-party tools or upload to a browser-based compressor like Lumin PDF. Lumin offers AES-256 encryption and meets SOC 2 Type 1, GDPR, and CCPA compliance standards for secure processing. For maximum privacy, use an offline app that processes locally.

How to Prevent Large PDFs in the Future

Prevention is better than compression. Resize and compress images before inserting them into the source document. Saving images at a lower resolution, such as 96–150 DPI, can greatly reduce file size, as noted by the Government of Canada’s IRCC help guide. For scanning, set your scanner to 150–200 DPI for text-heavy pages instead of the default 300 DPI.

Use standard system fonts like Arial or Times New Roman, which do not need to be embedded. If you must use custom fonts, enable font subsetting in your PDF creation tool. Most importantly, always use “Save as PDF” in Microsoft Office rather than “Print to PDF.” According to PDF Candy, the “Save as PDF” option from Office created a 0.5 MB file compared to 17 MB from “Print to PDF” for the same document.

Conclusion

Compressing a PDF is straightforward when you choose the right method for your platform, privacy needs, and file complexity. For a quick fix, use an online tool like Adobe’s free compressor. For offline control on a Mac, use Preview. For Windows, use the “Save as PDF” option in Microsoft Office or Adobe Acrobat Pro for granular optimization. Start with free tools that apply the six compression levers automatically—don’t settle for oversized files.

FAQ

Will compressing a PDF affect its quality?

It depends on the method and settings. Reducing DPI and re-encoding JPEGs can cause minor quality loss on images, but text remains perfect. Choosing the “ebook” setting (150 DPI) often yields no visible difference for screen viewing. For text-only PDFs, compression can achieve a 70% size reduction with zero visible loss.

Is it better to zip a PDF or compress it?

Zipping a PDF achieves only slight compression (~5–10%) because PDFs are already internally compressed. Dedicated PDF compression tools reduce file size by 50–90% by re-encoding images, subsetting fonts, and stripping metadata. Always compress the PDF directly rather than adding a ZIP layer.

How to compress a PDF that is password-protected?

Most online compressors cannot process password-protected PDFs. You must first remove the password using Adobe Acrobat or a dedicated tool, then compress the file, and finally re-add protection. Some desktop tools like PDF24 or Adobe Acrobat Pro allow compression of locked files if you know the password.