If it were my portfolio, I'd abandon the idea of PDF altogether and go straight to web. As you said yourself, people will likely be viewing it on a variety of devices, and a good, fully responsive design will do more to showcase your skills than a PDF that they have to wait to download. If the recipients want to print samples then they could decide for themselves how they do it. I think it's much more likely that a link would be shared around any interested parties.
Only my opinion though ...
Thanks, Malcolm, but I have certain images that I wouldn't want to publish publicly, but would feel more comfortable sharing only with potential employers, so I figured that a PDF portfolio might make more sense than a website. I am open to other suggestions, for instance a by-invitation-only website, some cloud based sharing, perhaps InDesign's new Publish Online feature. I don't know how to go about it.
Duke, I found your question when looking for the same answer. Since it's been about a year since you asked, did you ever come up with a suitable dimension for your purpose(s)?
I am creating a relatively simple interactive PDF (only navigation buttons and links to external URLs). I would like my PDF to be viewable whether using browsers, iPad, iPhone versions, or Android. I understand that that perfect fit for all platforms not possible with a single PDF. But, my main concern is that the menu is visible and not cut-off, no matter what device or computer is being used. So, a good, universal aspect ratio would be helpful.
Did you find it?