PDF text is rectangular & unreadable

Our Senior Center provides us with monthly pdf newsletters.
--- At their office those newsletters are readable.

They have a third-party company that sends them out via an email distribution list.
--- Until June 2018 they were readable.
--- However, starting in July 2018 something changed, the text is no longer readable as characters are in rectangular form.
--- The graphics for events & ads were still fine.


That's 4 months in a row now: I'd like to investigate possible causes.
--- Advice please
--- On those rectangular boxes, is there a technical name for them?

I can finally get back on this and I can see this is going to be any easy fix: as requested to what PDF viewer I'm using I don't have one. I checked a Windows 7 computer I hardly use, it has Adobe Acrobat Reader DC and the newsletter comes out just fine.

Usually I check the obvious but because the computer manuals I check on when servicing computers, the manuals are pdf's and are readable as expected: I don't know why right now but it is what it is.
--- Although I don't know for sure, I may have had Adobe Reader on my computer when upgrading from Win7 to Win10.
--- But it's probably my fault I eventually didn't have it anymore as I have experimented with a few different procedures on doing a fresh Win10 install: fortunately successfully in each case.
--- Many times with some of the people I help the best solution to get their computers up & running properly is to do a fresh Win10 install & those experiments prepared me for when a fresh Win10 install was the best solution on a troubled computer.

OK, so the moral of my story is always double check the obvious anyway even if something else is running ok.

I compared Adobe Acrobat Reader DC & compared it to Microsoft Store's PDF availabilities.
--- I'm staying with Adobe Acrobat Reader DC. Thanks Ken.