Why does Acrobat show a subset font name like OLXKFG+Calibri when selecting text?

In general terms, it means that the font is subsetted in your PDF file. 

Acrobat, and many other PDF generators, allow you to embed only the characters actually used in a PDF into the file, rather than the entire font. Depending on the document, that reduces the size of the PDF just a little bit (like it would in a book layout where you’re only using one or two font families to lay it out), or a lot (like in a newspaper or magazine, where every ad uses one or several different fonts). Subsetting your fonts also makes it more difficult to edit a PDF if you don’t have the same exact font — which is either a benefit or a problem, depending on your point of view and place in the reproduction process.

You may see just the font with one word, provided it uses the same font and weight in the document. But if you use bold, italic, or bold italic in that sentence, you increase the possibility that you don’t have all the characters of the font embedded in the document. But when you see those capital letters and a plus before the font name, it’s a solid indicator that you don’t have all the characters embedded in your PDF and if you don’t have the same font open in your system, you’ll likely have real issues if you try to edit it.

If you asked me, I’d say it’s worth mentioning to your students. You can use it as a cautionary lesson for editing PDF documents, and an excuse to emphasize that PDF editing can be perilous. The fastest way to create a useful edited file is usually to go back to the source document and create an updated PDF, including all the fixes made in your source document.

Hope this helps,

Randy