Prior to the Adobe Reader update to 18.09.14 I had no issues viewing and working with folders and PDF files in Dropbox on Adobe Reader.
On an iPhone 6s (iOS 12.0.1) with Adobe Reader (18.09.14) all Dropbox folders, and PDF, are visible.
On an iPhone XS Max (iOS 12.0.1) with Adobe Reader (18.09.14) only half of the folders are visible from Adobe Reader. It appears that recently created folders (created in Dropbox on a pc) are not being displayed in the Adobe Reader app on the XS Max, but displayed on the iPhone 6s correctly.
I had the same problem. I had to delete the acrobat app from my iPad and reinstall from the App Store. Then I could see my Dropbox files. Not sure how long it’s going to last though. The connection between Dropbox and acrobat is flaky at best on iOS.