Optimizing PDF files is crucial for efficient storage, faster loading, and easier sharing. Ghostscript (current stable version 10.05.1 as of April 2025), a powerful open-source interpreter for PostScript and PDF files, offers a robust command-line interface (CLI) that can significantly reduce PDF size, often without noticeable quality loss.

Introduction to Ghostscript CLI

Ghostscript is a well-established tool for creating, viewing, and transforming PostScript and PDF documents. Through its CLI, you can script or batch-process optimizations that would otherwise require bulky desktop applications.

Set up Ghostscript

Installation differs per platform:

  • Windows — Download the 10.05.1 installer from the official downloads page (choose gs10051w32.exe or gs10051w64.exe).

  • macOS — Homebrew keeps you current:

    brew install ghostscript

  • Debian/Ubuntu — Use APT:

    sudo apt update && sudo apt install ghostscript

Confirm the installation:

gs --version # Should output 10.05.1 or your installed version

Basic syntax for optimization

The fundamental command structure looks like this:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen \
   -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

Key switches explained:

  • -sDEVICE=pdfwrite — Specifies that the output should be a PDF.
  • -dCompatibilityLevel= — Chooses the target PDF specification version.
  • -dPDFSETTINGS= — Selects a predefined optimization preset (details below).
  • -dNOPAUSE -dQUIET -dBATCH — Ensures the command runs non-interactively without pausing or printing excessive messages.
  • -sOutputFile= — Defines the name of the optimized output file.
  • input.pdf — The original PDF file to be optimized.

PDF optimization presets

The -dPDFSETTINGS option provides convenient presets that bundle common downsampling and compression settings:

  • /screen — Lowest quality (72 dpi images), smallest file size. Ideal for on-screen viewing where size is critical.
  • /ebook — Medium quality (150 dpi images), balanced size and quality. Good for e-readers and general use.
  • /printer — High quality (300 dpi images), suitable for printing.
  • /prepress — Highest quality (300 dpi images) with color preservation, largest file size. Used for professional printing workflows.

A typical command for a good balance between size and quality using the /ebook preset:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook \
   -dNOPAUSE -dQUIET -dBATCH -sOutputFile=optimized.pdf original.pdf

Choose a compatibility level

Newer PDF viewers handle the latest features, but older devices or software might require compatibility with older PDF specifications. Ghostscript supports levels from 1.3 up to 2.0:

Level Spec Year When to use
1.3 2000 Maximum legacy support (Acrobat 4+)
1.4 2001 Safe default, enables transparency (Acrobat 5+)
1.7 2006 Broader feature set (Acrobat 8+)
2.0 2020 Smallest files via modern compression (Acrobat 10+)

Choosing the lowest compatibility level that meets your audience's needs can sometimes shave off extra bytes. Level 1.4 is often a safe starting point.

Step-by-step: shrink a PDF

Here are a few examples, starting simple and adding more control:

  1. Using the /ebook preset — A balanced approach:

    gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook \
   -dNOPAUSE -dQUIET -dBATCH -sOutputFile=ebook.pdf original.pdf

  2. Using the /screen preset — Prioritizing file size:

    gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen \
   -dNOPAUSE -dQUIET -dBATCH -sOutputFile=small.pdf original.pdf

  3. Custom settings — Manually controlling image resolution:

    gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.7 -dDownsampleColorImages=true \
   -dColorImageResolution=150 -dNOPAUSE -dQUIET -dBATCH \
   -sOutputFile=custom.pdf original.pdf

    This example specifically downsamples color images to 150 dpi.

Advanced optimization techniques

For finer control, you can combine presets with additional switches:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=2.0 -dPDFSETTINGS=/ebook \
   -dDetectDuplicateImages=true -dRemoveUnusedResources=true \
   -dCompressFonts=true -dSubsetFonts=true \
   -dNOPAUSE -dQUIET -dBATCH -sOutputFile=advanced.pdf original.pdf

What these extra flags do:

  • -dDetectDuplicateImages=true — Finds and deduplicates identical bitmap images within the PDF.
  • -dRemoveUnusedResources=true — Strips out resources like fonts or images that are defined but not actually used on any page.
  • -dCompressFonts=true — Compresses embedded fonts.
  • -dSubsetFonts=true — Embeds only the characters (glyphs) actually used from each font, rather than the entire font file.

Performance tips

Optimizing large or complex PDFs can be time-consuming. Consider these options to speed things up:

  1. Parallelize rendering: Use -dNumRenderingThreads=<cores> (e.g., -dNumRenderingThreads=4 for a quad-core CPU) to utilize multiple processor cores.
  2. Skip interpolation: Add -dInterpolateControl=0 if slight image aliasing is acceptable; this can speed up image processing.
  3. Disable transparency: If your PDF doesn't use transparency or if compatibility with very old viewers is needed, -dNOTRANSPARENCY might help.
  4. Batch processing: If optimizing multiple files, invoke Ghostscript once with multiple input/output pairs if possible, or script it efficiently to avoid the startup overhead for each file.

Example incorporating performance flags:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook \
   -dNumRenderingThreads=4 -dInterpolateControl=0 -dNOPAUSE -dQUIET -dBATCH \
   -sOutputFile=fast.pdf original.pdf

Practical use cases

Tailor your settings to the intended use:

  • Web delivery — Use /screen or /ebook, compatibility 1.4, and consider server-side gzip compression for further reduction during transfer.
  • E-readers — /ebook with font subsetting (-dSubsetFonts=true) usually provides a good reading experience.
  • Prepress — Use /prepress and ensure necessary options for color profile preservation (e.g., -dPreserveICCProfiles=true) are set if required.

Compare file sizes

Here's a sample comparison using a 12-page, image-heavy PDF:

Setting Pages Original Size Result Size Savings
/screen 12 8.2 MB 1.1 MB 86%
/ebook 12 8.2 MB 2.4 MB 71%
Custom (150 dpi) 12 8.2 MB 1.8 MB 78%

(Values measured with Ghostscript 10.05.1 on macOS. Results will vary based on PDF content.)

Licensing note

Ghostscript is dual-licensed under the GNU Affero General Public License (AGPL) version 3.0 and a commercial license offered by Artifex Software, Inc. If you distribute Ghostscript, distribute a modified version, or use it in a network service context where users interact with it remotely, ensure your usage complies with the AGPL terms or obtain a commercial license.

Common errors and solutions

If you encounter issues, check these common problems:

  1. gs: command not found — Ghostscript is likely not installed correctly or its location isn't included in your system's PATH environment variable. Verify the installation and PATH setup.
  2. Can't find (or open) font 'FontName' — The PDF uses a font not available to Ghostscript. Install the missing font system-wide, or try the -dNOPLATFONTS flag to prevent Ghostscript from using platform-native font rendering.
  3. **\*\*** Unable to open the initial device, quitting.followed byPermission denied** — Ghostscript doesn't have permission to write the output file. Check the permissions of the output directory.
  4. Invalid destination or similar file errors — Ensure the target path specified with -sOutputFile= exists and is a file path, not just a directory.

Wrap-up

Ghostscript's CLI provides a powerful and flexible way to achieve significant PDF optimization directly from the command line. By experimenting with presets, compatibility levels, and advanced flags, you can find the ideal balance between file size and quality for your specific needs.

If you need to integrate PDF optimization into a larger automated workflow without managing Ghostscript installations yourself, Transloadit’s Document Processing service leverages Ghostscript and other tools in the cloud.