We are occasionally getting the exact same problem. The problem files are Google Docs with charts pasted in (I am not currently clear on which paste method). This happens when students select the Google Doc from the file picker. It is my understanding that Google converts them to RTF before handing them off to Turnitin.
Our site has max file size set to 500 MB (for professors asking for video), and that's the same setting as for the test course I'm using. But, Turnitin says it has a 40MB limit.
Now, if I save the problem Google files as PDF they are are like 300K, but saved as RTF are like 322 MB - over 100 times as large. So, this part is a problem on Google's end. Interestingly, if I save as RTF (note: not Adobe PDF) and then upload to Turnitin, it works fine, despite the above comment about the 40MB limit.
It kinda looks to me like the plugin pays attention to the bogus 40MB limit when you're pulling from a Google repository, but not otherwise. Also, there's a known bug in the bug tracker related to the Assignment activity that throws the exact same error.
https://tracker.moodle.org/browse/MDL-62785
It is possible turnitin subclassed the Attendance module, and therefore inherited the same bug.
So, what I'm telling people is if they see the error message, save as PDF and upload the PDF.
Not a good outcome, but at least there's a workaround. Plus, it only affects a small minority of uploads.
Cheers!