Q: Does VoiceOver recognize tagged PDFs? Does it recognize alternate text tags in a  PDF?

I've been trying to find an answer to this problem as well, because it doesn't make sense to me why the answer is 'no'.

This article talks about the problem and is from 2013:

http://duff-johnson.com/2013/03/01/inaccessible-by-choice-pdf-on-macos/

As far as I know the problem remains in 2016; VoiceOver cannot recognize tagged PDFs (in the tech parlance this is often referred to as PDF/UA format).  I'm trying to find out when this will be fixed.

I too am having this issue.  It seems that VoiceOver does not read alt text for images on PDFs in Preview and just flat out doesn't work with Adobe Acrobat.  I would also like to know when this issue will be fixed or what Apple plans to do about it.