Please use PDF
I'm currently marking undergraduate Lit Reviews, and everytime I open up J. Bloggs' folder to see their work and see a gorram .docx file I actually want to cry. Word files are terrible generally, but I should also not be expected to fire up an entire office suite just to view a document. If I ever end up in charge of setting undergraduate coursework I'm making it a rule that every document should be submitted as .pdf where appropriate. Here's why.
(Nb: After this point I'll start referrring to file formats without the . and the italics since I'm writing in markdown and really can't be bothered to keep it up until I write a proper editor)
A .pdf file represents a finished product
A pdf file represents a finished document that you're willing to show the world. The whole point of a pdf is to "present documents in a manner independent of application software, hardware, and Operating Systems" (Adobe). It contains all of the information needed to display the document correctly -- typefaces/fonts, layout, sizes etc. Screensize not withstanding, I see a pdf the same on my machine as you do on yours. It even compresses it; I once had a 50MB OpenDocument file crunch down to barely an mb when exporting as pdf.
If you send in a docx file (or even odt or tex), that sends a particular message to me. That message roughly translates as "You aren't meant to see this yet". Those files are software-specific files for document creation and represent the toolkit you're using. If you were doing graphic design for someone, and handed in the work you would export it into a format that you knew they could see such as a png or svg. Obviously graphic design is a bad example, since you often send in source files as part of the hand-in, so that the client can mess up your work later on (or their team can tweak it for printing etc.). More accurately, if you were painting somebody a picture you wouldn't hand them a palette of wet paint. So don't hand in docx, tex, or odt files.
.pdf is an open standard
I could rant on about how awesome open standards are, but there are many who've done so already (and more eloquently then me) such as these guys. Adobe used to control pdf but they made it an ISO standard in 2008. There's still problems, obviously -- but being an open standard is the first step towards solving them for everyone.
It gives you the freedom to choose your toolkit
This one is probably the most important one for me as a technologist generally, but also having been through undergrad programmes (and currently doing postgrad stuff). I came to Uni as a fully-fledged Linux snob and couldn't use MS Word if I tried. I'd just completed an A-level in Double ICT which involved a lot of document creation, so I knew first-hand how terrible doc and docx files were to use in other office suites. Opening a document in LibreOffice that was created in MS Word is one of the most painful experiences a F/LOSS user can ever go through due to formatting errors (although I'd argue that's docx's fault rather than Word or Writer, but that's another blog post).
If you're working collaboratively on a document, you should have a discussion about tools used for document creation. This is important inside and outside of academia (I had an experience recently with LaTeX-Word conversion on a paper). If you're working on your own, however, producing a pdf as a final product means that you can choose whatever system you want to produce it in and nobody else is affected. I began producing all my reports in LaTeX a little while ago, but I can also jump back to a word processor whenever I want without much change in output at the other end. I can even use PHP to turn a web page into an A4 pdf report if I so choose and achieve the same results.
Whatever your opinions are on the pdf standard, simply having a standard that represents a finished product is a good thing. You should be handing that in whether it's a CV/resumé, a short-form report, or a thesis. How you produce it therefore becomes a relatively minor issue, and should be invisible on the other end. Pdf is how we should achieve this short-term, and then we can begin fixing that standard once we have that mindset.