Breakthrough! with Adobe, that is, but along the same lines as Stella describes with (I assume) Foxit.
I've been trying to figure out how to get the 8.1 OS on my tablet to read PDFs to me since I got a lenovo yoga tablet last year. Never had a problem reading documents out loud in Adobe with Windows 7, but couldn't do Read Out Loud with any PDF
once Windows 8.1 entered my life. With the tip that it might be something in Adobe that needed changing (despite logical reasons for assuming something needed tinkering/toggling/whatever in Windows), I fished around in Adobe help and found a reference
to a conflict between accessibility features and Protected Mode.
In short, to make Adobe XI read out loud in a Windows 8.1 environment, go to Edit: Preferences: Security (Enhanced) and deselect "Enable Protected Mode at startup." Taking the check off that box removes the default "Protected
Mode." Then exit Adobe, go back in, and listen to Microsoft David (or Hazel or Zira) read to you!