Downgrade PDFs on the fly for iOS users who lack the memory to display the PDF.

[mkdir-washington-edu]

The problem
In LMS assignments (and likely non-LMS readings) instructors might use high-resolution PDFs, but due to limitations on iOS devices with insufficient memory the text of the those PDFs won't show up in the LMS reader.

Related issues:

• Some PDFs on iOS are showing as blank support-legacy#178
• Investigate resolving issues with high-resolution PDFs on iPads #1278

Our current solution is to send instructors to use this online Adobe tool which compresses the PDF without changing the fingerprint, but this is still cumbersome for some instructors because ultimately they don't know a PDF has an issue until after students report it, and it requires they edit their assignment.

The solution
Using a guide to likely memory storage limits (I would recommend erring on the smaller side, like older iPhones and iPads) detect if a PDF the instructor is going to use surpasses those limits. At that point we can at least display a warning and remediation steps, though a better solution would be to create a new document and point the assignment to that document instead.

Related issues (though about OCR, not document size):

• SPIKE - Detect whether or not a PDF has a digital text layer prior to the first LMS app launch  #1225
• https://github.com/hypothesis/private-issues/issues/42