Firefox does not use the native Android download manger. It uses its own. The download folder is /sdcard/download/.

I think on most devices Firefox for Android will download to the system download folder. You have to enter some obscure settings into Firefox to change that.

Are the PDFs visible on Firefox's download list:

menu > Tools > Downloads

If they disappear between sessions, you may have Firefox set to clear data at exit. You may want to turn that off under:

menu > Settings > Privacy > Clear on Exit

If you want to clear some data but not lose downloads, make sure the box for Downloads is NOT checked.

Thank you - that's the first time I've seen that folder. However, there's 3 versions of the same pdf (understandable) and a load of inexplicably named files marked as Android, e.g. 9dysoARb Unknown size - tearfund.org (source website) Cancelled

I notice there's an OS update due so perhaps these are failed downloads?

Richard