How do I import a PDF that's on my device or SD card?

On Android 4.4 and later (including Chromebooks), Squid uses the Android system user interface (document picker) to select a PDF. The first time this is launched, the internal device storage and any external storage such as an SD card may be hidden:

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:

For a video tutorial about importing PDFs, see Squid - Working with PDFs.