So, time to drop this question again before it’s too late – for a product that is PDF and print-on-demand, what type of font do you prefer – serif (i.e. Times New Roman) or sans-serif (i.e. Arial).
In an ideal world, a PDF would be a PDF, but I have found out the hard way that no matter how many platforms a file looks fine on, someone will be using some combination of hardware and software that will scramble something. I have therefore reluctantly concluded that compatibility and simplicity is king, so it’s down to those two industry standards which will work anywhere.
So, the question comes down to whether you prefer your letters with squiggly bits on the ends, or not. Over to you.
Dreamscape Design 
I have no answer for such a technical question (I’m not good enough to know that), but I’m sure that Bob Marley wouldn’t like the Serif.
“I shot the Serif, but I didn’t shoot the Deputy!”
Well, if Bob says so, it’s got to be sans serif! Helvetica, I’m looking at you.
See, here’s the problem with multiple platforms of publication:
Serif fonts are made to be easier to read in print. Sans Serif fonts are made to be easier to read on a screen. If you use both digital and print format, you’re going to be using a a font for one that isn’t optimal.
A better question to ask is how many people plan to print off the pdf or use it on a tablet: if more people plan on using it on a tablet, go for San Serif.