just store the pdf file in the ASD - application specific directory, for details please search the community
after App Inventor targets SDK30 it looks like we will not be able to store files to a directory on the internal (emulated) sdcard anymore except of the ASD
the MIT App Inventor team still did not decide if we will be able to store something to the shared storage then
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.