How do I import a PDF that's on my device or SD card?
To show the device storage and any external storage, tap the three dots in the upper right corner:
Then tap "Show internal storage" (or similar; your device may show something different):
Then tap the three horizontal lines in the upper left corner to open the drawer again:
You will now be able to select the internal device storage as well as any external storage such as an SD card or USB drive that is attached: