Texpad registers itself with the iOS device it is installed on as a candidate app for opening LaTeX-related file types such as .tex, .bib, etc. This may be used to import a file into Texpad. For instance, if you open a file in your Dropbox app on iOS, you see an Export button,
Tapping it will bring the following dialogue
Here you may select Open in…, which will show Texpad as one of the options
Files imported in this way will be copied to the Texpad-Imported-Files folder in the root of your local iOS storage.