As you are trying to open the PDFs from Dropbox, you can add Dropbox account in the applicaiton itself.
Launch the Adobe Acrobat Reader and go to Files > Location > Dropbox. Click on Sign in to add the Dropbox account in the application. Then you can directly open the Dropbox PDF files in Acorbat Reader.
If you wish to open files directly from the Dropbox application on your device, refer to the steps provide dbelow:
- Open the PDF in Dropbox application.
- Click on the three dots provided at the top right-hand corner of the application.