How can you get a local filesystem link of an offline-accessible PDF file in Dropbox?
It's in 

YourLocalStorage/Android/data/com.dropbox.android/files/uXXXXXX/scratch/Your/Folder/In/Dropbox/file.pdf


Note that YourLocalStorage depends on your mobile. For example, it is storage/emulated/0/ on some LG phones.

To access the file from your web browser you have to put file:/// before the given path.