A scanned PDF from a MacBook Air or Pro can easily reach tens of megabytes, which is too big for an email or an upload form. macOS has a way to shrink it in Preview, and a second one in ColorSync Utility. Both are free and already installed. They also have limits, which this guide points out so you can pick the right route.
Method 1: Preview's Reduce File Size
Apple's Preview guide describes this route for PDFs:
- Open the PDF in Preview.
- Choose File > Export (not Save).
- In the Quartz Filter pop-up menu, choose Reduce File Size.
- Click Save. If Preview asks whether to replace the existing file, click Save to overwrite it, or click Cancel, type a new name in the Export As field and click Save.
Saving under a new name is the safer choice, because it leaves the original untouched. Apple's page also lists three checkboxes in the same dialog: Create Linearized PDF, which optimizes the PDF for fast loading in web browsers; Optimize images for screen, to reduce file size; and Save images as JPEG, which converts images to JPEG format with image compression.
The drawbacks of the Preview route
Apple states one caveat plainly: when a PDF is compressed, it may be of lower quality than the original. The resulting file size and image quality depend on the options you select. Apple does not say how much smaller the file will be or how much quality is lost, so open the result and read a page at normal zoom before you send it.
The steps Apple lists also have no field for a target size. If a form asks for a file under 1 MB or 200 KB, the only way to find out whether the filter got there is to check the size in Finder, and to try again with other options if it did not.
Method 2: a custom filter in ColorSync Utility
The Reduce File Size filter is one Quartz filter among others, and ColorSync Utility lets you build your own. Per Apple's ColorSync Utility guide:
- Open ColorSync Utility and click Filters in its toolbar.
- Click the Add button in the lower-left corner, name the filter and press Return.
- Use the Edit pop-up menu beside the filter's name to choose a component. For Image Effects, Apple says you can change the size, bit depth, interpolation and compression settings for the images in your file.
- To use it, choose File > Open and select the PDF, pick the filter from the Filter pop-up menu at the bottom of the window (the preview changes, but the file is not modified yet), then click Apply.
Apple notes that a locked filter cannot be edited directly: click its Duplicate button and edit the copy. Apple's page says that Quartz filters can modify a PDF by reducing the size of the file, but it does not say which settings give a given size. That means trial and error, and ColorSync Utility is the more fiddly of the two routes.
Method 3: DocPro Compress PDF in Safari or Chrome
A browser tool avoids both limits: it has a quality choice and a target size. It works the same on a MacBook Air, a MacBook Pro or any other Mac, in Safari, Chrome or Firefox.
- Open Compress PDF in your browser.
- Drop the PDF on the page or choose it from Finder.
- Pick a quality level: High quality, Balanced, Small or Smallest. Or tick Aim for a target size and type the number of KB your form allows.
- Click Compress PDF, wait while each page is processed, and read the Before and After sizes.
- Save the new copy. The original is not changed.
For common limits there are ready-made pages that open the same tool with the number filled in: 1 MB, 200 KB and 2 MB. For the reasoning behind choosing a target and what to try first, see how to compress a PDF for email and upload limits.
The file is processed in the browser on your Mac and is not uploaded to a server. That matters for scans of IDs, contracts or medical letters.
What DocPro's compression costs you
- Each page is re-drawn as a picture and saved in a new PDF, so selectable text and links are lost, and the result cannot be searched.
- With a target size the tool lowers quality first, then resolution. A very small target across many pages gives soft, hard-to-read pages, and it may not reach the target. The tool reports the size it reached.
- If the result is not smaller than the original, the tool keeps the original and says so.
If the document is mostly text with no pictures, it is already small, and picture-based compression may not make it smaller. In that case, split out only the pages you need, or remove blank pages, or use grayscale for black-and-white scans.
Which route to pick
- Preview is quickest when you only need a smaller file and do not mind checking the quality.
- ColorSync Utility suits people who want to control image settings on the Mac itself.
- DocPro suits a hard limit, such as under 200 KB, because it lets you type the number.
Keep the original file. Any compression trades detail for size, and you cannot get the detail back.
Frequently asked questions
Where is Reduce File Size in Preview?
Open the PDF, choose File > Export, then choose Reduce File Size in the Quartz Filter pop-up menu and click Save, according to Apple's Preview guide.
Does reducing the file size in Preview lower the quality?
It can. Apple says that when a PDF is compressed, it may be of lower quality than the original, and that the resulting size and quality depend on the options you select.
Can Preview reduce a PDF to a set size such as 1 MB?
Apple's steps do not include a target size. They offer the Reduce File Size Quartz filter and checkboxes for Optimize images for screen and Save images as JPEG.
What do the Optimize images for screen and Save images as JPEG options do?
Apple describes them as optimizing images for screen viewing to reduce file size, and converting images to JPEG format, which uses image compression.
Can I make my own PDF filter on a Mac?
Yes. In ColorSync Utility you can add a filter under Filters and, using Image Effects, change the size, bit depth, interpolation and compression settings for the images in your file. Apple says a locked filter has to be duplicated before you edit it.
Will Preview ask before it overwrites my PDF?
Yes. Apple says Preview asks whether to replace the existing file when you export. Cancelling and entering a new name in the Export As field saves a separate copy.