Compressing PDF Images with macOS


Did you know you can (sometimes) shrink the size PDF files by apply lossy compression to images? This is a built-in capability with macOS - no third party tools required. Read on...

PowerPoint PDF Compression Setting

Before going on, if you're using PowerPoint then there is a Low quality setting for the smallest PDF size:

  1. Under Preferences > General select LowPrint Quality (Paper/PDF).
  2. Then File > Save As... or Export... and select PDF as the File Format.

PowerPoint Preferences to set PDF Print Quality

It's not so easy with Word - you can Compress Pictures from the Picture Format ribbon, but that means you'll loose the original high-quality image.


Printing to PDF + Preview Export

For Word:

  1. First, Save As...File FormatPDF,
  2. Then use Preview to further compress the PDF. Open the PDF in Preview, File > Export..., select Format as PDF and Quartz Filter to Reduce File Size.

For other apps that support printing, to force highest (lossy) compression of images, do the same:

  1. First "Print to PDF" i.e. from the Print dialog, select the PDF drop down > Save as PDF,
  2. Then use Preview to further compress the PDF. Open the PDF in Preview, File > Export..., select Format as PDF and Quartz Filter to Reduce File Size:

Preview Export as PDF with Reduce File Size Quartz Filter


Automator Print Plugin to Compress PDF

If you don't like the two step process, then create a simple Automator script!

Option 1 with "Compress Images":

  1. Run Automator.
  2. Under File > New, create a new Print Plugin:

Automator New Print Plugin

  1. From the left hand Library:
    • Drag-and-drop Compress images in PDF Documents and select Compression as JPEG and QualityLeast.
    • And under it, drag-and-drop Move Finder Items to Desktop.
  2. Save it, e.g. as "Save as Compressed PDF".
  3. Now, just go to the Print dialog and you'll see a new option to "Save as Compressed PDF".

Automator Compress Images in PDF Documents

Option 2 with "Quartz Filter":

For even smaller files, with even worse image quality, replace step 3 above with Apply Quartz Filter to PDF Documents, select FilterReduce File Size followed by Move Finder Items as before:

Automator Apply Quartz Filter to PDF Documents to Reduce File Size

Save it as "Save as Reduced PDF" and now, the print dialog PDF drop-down will show both options:

I assume Reduce File Size simpy performs both:

  • Image Sampling - scale to 50% (max of 512x128 pixels), and
  • Image Compression - JPEG quality medium
A Quick Test

I did a short test with horribly large PowerPoint file filled with pictures. A 526 MB PPT got shrunk down to:

  • 5 M - Export High quality, followed by Reduce File Size
  • 2.5 MB - Print Low Quality, followed by Reduce File Size

It's impossible to recommend what would work for you... it depends on the type of images you have (screenshots / images with text vs background images). I have to eyeball and select based on what file size out to achieve.

At least you know you can Export Low quality and then apply the reduce file size quartz filter. Then, do what I do - manually replace any illegible slides from a decent PDF to this one, by dragging-and-dropping from another PDF in Preview.

Did you know you can move, add, delete pages from a PDF in Preview? Fantastic!