A few days ago we had a problem with viewing the pdfs online. Some letters moved and coupled. We uploaded the pdf several times thinking we did it, but it turns out that the problem was another. Thanks to @Vitaliy for helping us. We share your solution in case it is useful to more users.
BlockquoteIt looks like PDF.js problem; probably related to this issue: pdf.js does not display ff characters · Issue #4749 · pkp/pkp-lib · GitHub
Try to update OJS to the latest version or to release, where this problem is fixed.
Greetings and happy holidays!