Reducing PDF image size on OS X

How you can greatly reduce the size of PDF files using stock applications that come with OS X

In this guide we’re going to greatly reduce the size of some PDF files using the Preview application. The built in application for previewing files on OS X.

If you’re on Windows or Linux there are almost certainly things you can do, but unfortunately this guide isn’t for you. If you’re on Mac OS X and want to make your PDF files smaller, read on!

The quick version of this guide is;

  • Open the PDF in Preview
  • Click File > Export
  • In the Quartz Filter section of the window that appears select “Reduce File Size”
  • Save it somewhere you can easily find it
  • Compare new to old

Lets go into those steps in more detail.

Open the PDF in Preview

If you haven’t got an application specifically for reading PDFs then double clicking on the PDF file or typing open my.pdf from a terminal will open the PDF in Preview. If you aren’t sure you can always open the Preview application first and then click on File > Open and browse to your PDF.

Export the PDF

This will sound counter-intuitive, because you normally export when you want to create a different type of file, but bare with me!

When you go to File > Export a new window will open, the first thing you want to do is go down to where it says “Quartz Filter” and in there select “Reduce File Size”. There are some other interesting options in there, but we can save those for another post.

Now what I would suggest is either renaming the file, or saving it somewhere other than where your original file was. This is important because once we’ve created this smaller file we want to check it to make sure we’re happy with the results.

Check the output

When you are doing any sort of compression or file manipulation it always pays to look at the output. What we’re checking for is that there are no pages that look poor quality as a result.

I’ve been using this method to create smaller PDF files for a while and rarely notice any visible change at all, but it is always worth checking.

Even if you do notice some slight change, you need to ask yourself if the quality you’re now seeing is fit for purpose. I’ve seen people open up files on huge iMac screens zoomed in to 300% and complain about some slight lack in quality. Unless you heavily suspect your end reader will be doing the same, it is fine!

Big savings

One of the reasons I’m excited to share this tip is because the savings you can get are non-trivial. This isn’t just shaving off a couple of kilobytes from the file size. I was recently working with one of my favourite clients and was able to reduce a PDF they needed from 39.5mb to 5.6mb without any visible difference in quality. This makes a big difference to file upload times, download times, and over time storage and bandwidth costs.

PDFs aren’t the only thing worth shrinking

I’ve written up some thoughts on image compression as well, which you may be interested in reading.

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