Hi there, Seaster.
You can attach PDF documents in QuickBooks Online for Mac as long as the file doesn't exceed 20 MB.
If the file doesn't exceed the limitation, we can isolate the issue by trying the basic troubleshooting steps.
First, try opening a private browser. This is the best way to check if it's a browser issue. If it's working using the incognito window, try to clear the browser cache. Too many files may affect the browsing performance and may require you to clear it out.
In addition, using a different browser can help fix issues like this (Internet Explorer, Google Chrome, Safari, or Mozilla Firefox).
Don't hesitate to reach out if you need more help with this. Happy holidays!
