Some PDFs on iOS are showing as blank

We have received occasional reports of iOS devices not showing any text on a PDF in the Hypothesis viewer. See:

- https://app.hubspot.com/contacts/6291320/ticket/245053009/
- https://app.hubspot.com/contacts/6291320/ticket/298760738/
- https://app.hubspot.com/contacts/6291320/ticket/327827814/

PDFs work perfectly fine using desktop browsers, but on iOS devices will render as below:

PDF renders badly in iOS Safari and Chrome. Forcing a new OCR on the PDF using https://docdrop.org/ocr/ does not change behavior either.

I was able to get a working PDF in iOS by exporting the original to TIFF, then re-exporting as PDF, finally running the document through OCR again. Essentially creating an entirely new PDF from image.

**To reproduce**
On an iOS device, open the following assignment: https://hypothesis.instructure.com/courses/92/assignments/1475