What, to a normal person, is the easiest font to read, in a pdf, on a computer screen? This document will not be printed so ink saving is totally not an issue. I’m only concerned with eye comfort and aesthetics.
Papyrus, or comic sans.
Actually, lots of people like Helvetica.
For some unknown reason, Helvetica is not in my font list.
Personally I love Papyrus but if I had to read more than one page of it I’d possibly lose what little mind I have left. 
The new default fonts by Microsoft are engineered to be read on-screen, so you can just go with one of those (Calibri, Candara, Cambria, COnstantia, etc). You can also use Verdana or Trebuchet (also defaults) as those were also engineered for screen use.
I’m pretty sure that Arial comes on most Windows computers, and Helvetica doesn’t. They’re quite similar in look and usage, and unless you’re a graphic designer or something, they’re virtually interchangeable. I personally prefer Calibri to both as my generic go-to sans-serif font.
Verdana and Georgia were designed for on-screen reading.
Please don’t choose Times New Roman. It’s really not the best for reading on-screen.
Frankly, as a graphic designer, I love Georgia. I was not sure whether it was screen-designed so I didn’t recommend it off the bat, but since it is, I’d go with that (at least for either body or title text; do a sans-serif for one and a serif for the other for more interest!)
Arial is a substandard knockoff of Helvetica, but Helvetica costs money for Windows users so it’s a grin and bear it situation.
Knuth Computer Modern.
Since you’re sticking with fonts you already have, we’re fairly limited.
I’m still not a fan of serif fonts on screen. (It’s the opposite of print, as I only like it for small snippets like in headings.) Everyone notes that Arial is a lesser Helvetica clone, but people forget about MS Sans Serif, which is also one. If you don’t like Arial, try that one.
If you like what the Dope uses for posts (on Windows), it’s Trebuchet MS. If you prefer what it uses for the text boxes, try Verdana. Both were definitely designed to look good on screen. (Personally, I’d go with Verdana, since Trebuchet MS has problems with lowercase w’s at small font sizes. Though maybe not in Adobe Reader, which uses its own fancy font handling that might fix it.)
Yeah, if you’re going to use a serif font, use Georgia. I’m currently using a Linux USB instead of Windows while I wait on parts to fix my computer, and I’ve not bothered installing the Windows fonts, as the default Deja Vu Sans is fine. But I think I am going to go ahead and install Georgia. It’s much better than the Times New Roman clone that comes with the system.
You’re mean. 
I like Georgia or Garamond.
I went to try out Georgia to see how I liked it. It’s good, but I still like Century Schoolbook for ease of reading on the screen. It’s very much like Georgia, but spread out a little bit more.
Frutiger® is nice, but it costs money. I like Calibri just fine for my daily e-mail and whatnot. I have always liked Garamond for printed material when I want something a little classier than the standard sans serif stuff.