How to Make a File Smaller on Mac
When sharing a PDF file via email or keeping it in cloud storage, it makes sense to keep the file as small as possible so you don't waste space. On a Mac, you can use the built-in Preview app to resize PDFs, which is convenient.
However, this process isn't perfect as it drops the file quality significantly. We'll show you how to reduce a PDF's file size without losing quality on a Mac.
How to Resize PDFs in Preview on Mac
It's easy to lower the size of a PDF using Preview. To do so, just double-click a PDF in Finder to open it in Preview. Once it's loaded, select File > Export from the menu bar at the top of the screen. In the resulting window, open the Quartz Filter dropdown box and select Reduce File Size before exporting the file, which will make it smaller.
While this provides a lower file size for the PDF, the problem with this method is that it drastically reduces the quality of the file. If your PDF becomes so blurry that is illegible, that isn't a suitable solution.
Thankfully, you can tweak some files on your Mac to strike a better balance between quality and file size. However, this requires disabling an important security protection on modern versions of macOS.
Disabling SIP to Tweak PDF Export Quality
Starting with OS X El Capitan in 2015, Apple introduced System Integrity Protection (SIP). This increases the security of your Mac by restricting access to sensitive folders on your device. Unfortunately, you need access to the protected /System folder to perform this PDF tweak.
If you're comfortable with it, you can disable System Integrity Protection to proceed with the following steps. If you do this, make sure to enable SIP again as soon as you're done in order to keep your Mac safe. You should also avoid downloading any untrusted software while you have SIP turned off.
In case you don't want to go through these steps, there are easier ways to reduce a PDF's file size without losing quality. Web tools are your best option; try a free service like SmallPDF to reduce your PDF's file size without downloading anything.
How to Reduce PDF File Size on a Mac Without Losing Quality
To tweak the necessary system files after disabling SIP, first open Finder. Select Go > Go to Folder from the menu bar, then enter the location /System/Library/Filters.
The contents of the resulting folder control the options available in the Quartz Filter dropdown mentioned earlier. You should see one called Reduce File Size.qfilter. Copy this and paste it to your desktop or another convenient location.
To give yourself multiple PDF compression options, create multiple copies of the Reduce File Size.qfilter file. By changing the below values differently for each one and saving them all separately, you can have access to various compression levels from the same menu.
Editing the PDF Quality File
Now, right-click on the duplicated file you just created. Choose Open With > TextEdit, or whatever your preferred text editor is. It opens as an XML file, which is a common format for saving program options in a way that both humans and machines can read and understand.
Inside this file, look for the Compression Quality line, which is inside <key> tags. You can press Cmd + F to search for it, if needed. By default, the value below this line, in <real> tags, is set to 0.0. However, you can set this to any value between -1 (highest compression, lowest quality) and 1 (least compression, best quality).
You may need to change this value a few times to find the right balance of quality and file size for you. Try 0.5 for medium-level compression—you can adjust it later if it doesn't seem right. If you're making multiple files for various levels of compression, try 0.75 for a higher-quality file.
Still inside the XML file, the next field you should check is ImageSizeMax. By default, this is set to 512, but you can change it to increase the final size after compression. Try setting this to 1684 for medium-quality compression (resulting in A4-size paper at 144DPI), and 3508 for high-quality compression (equivalent to A4 paper at 300DPI).
Finally, look for the Name field at the bottom of the file. Change this to something clear, as the name will appear in the Quartz Filter dropdown box when you're done. If you're making two new files, you could use Reduce File Size Better for the medium-quality one and Reduce File Size Best for the high-quality option.
Adding the New PDF Export Options to Preview
Save each file when you're done. Rename the files in Finder to match the names you gave them in the XML, then copy and paste them back into /System/Library/Filters where you got the original file from. If this doesn't work, make sure you've disabled SIP, as you can't move files into this folder when it's active.
Now, when you use the Export menu in Preview, you'll see the new options for compression in your Quartz Filter dropdown menu. Use them to create a smaller PDF that doesn't drop as much in quality as the default.
If you don't like how it looks, or the file size is still too large, try playing with the values a bit until you get them right. Remember to turn SIP back on once you're satisfied with your new export options.
Compress Mac PDFs Without Losing Quality
Now you know how to reduce the file size of a PDF in macOS without making it completely blurry. Disabling SIP is an intensive step, so you might not want to do all this if you only need to resize PDFs occasionally. But if you use the option in Finder all the time, having the better-quality options right in the same menu is handy.
Don't forget that there's a lot more you can do with PDFs on your Mac!
Image Credit: Sasun Bughdaryan/Shutterstock
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About The Author 
Ben Stegner (1789 Articles Published)
Ben is a Deputy Editor and the Onboarding Manager at MakeUseOf. He left his IT job to write full-time in 2016 and has never looked back. He's been covering tech tutorials, video game recommendations, and more as a professional writer for over seven years.
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How to Make a File Smaller on Mac
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