This PDF displays with question marks instead of letters
It looks like all the previous answers to this question are completely uninformed. I suggest that you completely disregard all of them. It is unlikely, for instance, that telling the document creator that what they did was faulty will really get you anywhere.

First, it's likely that they did not embed fonts, yes, but there are many fonts that carry flags telling apps NOT to embed them. This is an especially favorite trick of Microsoft products. Presumably they do not want people to be successful using non-Microsoft products (at least, that's what the documents in their antitrust case indicated), so they make it as painful as possible to transfer documents from Windows to Mac. Other companies know about this, and they may try to compensate. For instance, Adobe may license fonts to make the process more transparent to the end user but Apple may not have the same license for Preview. There is a lot going on behind the scenes. "Correctly" embedding the font won't get to the root of the problem either.

Second, you say that this document was created by someone "already not very good with computers." It would be ridiculous to tell such a person to correctly embed fonts. It's not even fair to consider the lack of font embedding as an error since you could quite possibly install the fonts, depending on the license, in a place where Preview can see them.

Third, it is naive to claim that Preview is at fault because it can't find a font when you have not told it where to find the font.

You can compensate for this. I can think of at least three ways.

First, if the creator wants to embed fonts, they may be able to and they may be able to choose similar fonts with different licenses if that is the source of the problem. In the displayed example, there is a correctly rendered font in the lower left hand corner. The creator could possibly use that font throughout.

Second, if you don't want to go to the document creator, you can apply one of several software solutions. I prefer open source software so, when I receive a file like this, I use the free, open source ps / pdf tools you can get with Xpdf. The problem with this solution is that many people don't want to use open source or feel uncomfortable with it. There are projects to make it easier on Mac. I use a package installer called macports to make it easier for me. Having done that, I would solve this problem by typing the following two lines in the terminal:

pdftops badfile.pdf intermediatefile.ps
pstopdf -o goodfile.pdf intermediatefile.ps


This will effectively replace the non-embedded fonts in badfile.pdf with their nearest Postscript core font equivalents. For me this is usually adequate. If the image in the above picture is going to be part of a book, you may need the fonts to match. In this case, you must either learn more about fonts or purchase non-free software. Depending on the circumstances, the options to the commands mentioned above may be modified to match fonts precisely.

Third, if this is a book project and you are uncomfortable with free software, you should try using a combination of Adobe Acrobat (not the free Adobe Reader) and Adobe Distiller to convert the document from pdf to ps, put the desired fonts in place, and convert the document back to pdf. I just googled and saw that there are a number of hits for tutorials for doing this process.

By the way, as a former professor, I feel that students who don't "try and get them to change their ways" are not doing me or the other students a favor.

Sometimes there are legitimate reasons why professors resist change. I don't know about your cases. If the professor is "already not very good with computers" you might want to use psychology. Since you are in a college or university, there are probably some experts around who could help you. Maybe the prof feels threatened or confused in an area outside his expertise. Maybe he's afraid students will think he's generally stupid. Who knows? ... But you might be able to figure it out and help everyone in the class. Good luck!