Now that I've started out with my final choice, let me talk about the other three that I was playing with (Folia, Google PDF Viewer, and Adobe Acrobat Reader for Android) and explain why I found them wanting.
Folia was the original iAnnotate app for Android, but almost from the beginning, iAnnotate for Android was seriously lacking as compared to its iPad counterpart. Eventually, iAnnotate for Android became Folia and was intended to be more comparable to the iPad app. In fact, it was mean to go above and beyond the app and essentially be a whole new ecosystem for managing documents. And therein was the problem. If I was already heavily invested in Google Drive (I actually pay for storage) or Dropbox (my linked PDFs for Zotero are stored in Dropbox) or another cloud storage service, why would I want to switch? Moreover, while the Folia architecture may be great for linking different documents, I've been leaning more and more toward a service such as Google Drive and increasingly One Drive where I could actually create documents, not just read and annotate documents. Folia also has a 20 document limit for the Free account. If you want more files in Folia, you'll have to pay $1.99/month. I have hundreds of PDFs-- I just didn't see myself paying for Folia storage (not to mention that I had already paid for the iPad version!). Finally, the most unforgivable sin, according to Folia's FAQ, you had to export each pdf in order to see markups made with Folia:
Can I see my annotations outside of Folia?
Annotations made in Folia are designed to stay in Folia. It is where they will be the most powerful. With the introduction of Folia for iOS and Android and our Folia web app, you'll be able to view your documents on iOS, Android, PC, or Mac. You'll be able to annotate them on mobile - annotation on the web will be coming soon. So, regardless of what device you are using, you'll be able to get the best experience.
You will also be able to export your documents as PDFs to view outside of Folia. One of the reasons we are present on most platforms is so you can enjoy Folia to its fullest, so it will be our recommendation that you keep documents within the Folia environment.In other words, I would have to import all my documents into Folia and if I wanted to see my markups in anything other than Folia, I would have to export them one by one...Can you imagine doing that for all the pdfs you might read? (again, I'm a researcher, I read a lot of pdfs) Finally, I also didn't find reading PDFs that great in Folia for Android. For one thing, it didn't have a lock screen function (which allows you to resize a PDF and freeze the resizing for easier reading.
Google PDF Viewer
This was a useful app to allow you to open PDF's, but it was too simplistic-- you could do exactly what the title implied, view pdfs, but no annotation.
Adobe Acrobat Reader for Android
This app was pretty good for reading and annotating PDFs and while the app itself is Free, but if you want to sync the document and annotations, you'll have to use Adobe's Document Cloud which it pricey. Like Folia, if my workflow was already set up with Dropbox or Google Drive or OneDrive, why would I want to pay for Adobe's Document Cloud? Here's a review of Adobe's Document Cloud. And to be honest, I've always hated many things Adobe-- the cost, their digital editions (awful, awful experience with books I paid for!), and the fact that a few years ago, they were hacked (the first time I received a letter saying my private information had been exposed).