Can't open PDF files with new beta on my mac

Thanks for the reply, Megan. I mean that when I double click a pdf or jpg to normally open it from the Dropbox folder in a Mac finder window, the icon quickly expands as if the document is going to open a second later, but it never does. From that finder window, I must right click on the icon to open up another menu and tell it to open with Adobe Acrobat in order to get it to open. A simple double click that's worked for decades no longer gets the file open.

I'm trying to replicate the dialog box that that tells me that it won't open because the file is from an unidentified developer, but it won't show me that right now. 

I've identified something else that's happening which I think is the root problem. When I right click a pdf and select "Get Info," the application that shows to be the one that's trying to open the pdf is Dropbox, not Adobe Acrobat. The same happens when I try and open a Word document stored on Dropbox. When I click the Dropbox selection, other apps appear that I can choose to be the default app to use for pdfs. Acrobat is listed and it surprisingly says, "default" next to it.... yet when I look at the Get Info box, "Dropbox" is already populated as the app it wants to use to open the file. Then, when I change it to open all files like that (meaning all files with the extension ".pdf"), it doesn't do that. I have to go into each individual file, open its Get Info menu, and tell it to use Acrobat to open it if it's a pdf or Preview to open it if it's a jpg.

The bottom line is that Dropbox really, really wants to be the app to open all my files even though Preview is the default jpg viewer and Acrobat is the default pdf viewer. That's despite the fact that I'm setting my preferences to make other apps the default to open those files.