What os version are you using on your Galaxy 4 android device?
In order to locate your PDF, you might tap or the red button on the lower right corner of your recents tab, and search for your PDF using the search option available on the top, if you know the exact name of your PDF.
Alternatively, you may go to the Local tab and locate your PDF in the list of PDFs that you see.
If you are unable to locate your PDF using either of the options above, you might want to install a file browser on your phone and check if the PDF actually exists on your phone or not.
Adobe Acrobat Team