Why does office online server open pdf files with black background?

Question

Bitrix24

Good afternoon!

There is a portal deployed on Bitrix24. An office online server is connected to it for viewing files.
Recently there was a problem with displaying open pdf files.

The essence of the problem:
PDF files are automatically opened in word. And some of them (not all) open with a black background.
Moreover, if you select the text and copy it from the file, then it is there.
At the same time, if you click on print a file from word, the preview will show it as it is without problems.

I have an assumption that when opening the file, it is converted from pdf to doc, but since I'm not good at configuring things like this, so I can't be sure.

Maybe someone faced such a problem?