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, ios-export-button in the bottom left corner,

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Tapping it will bring the following dialogue

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Here you may select Open in..., which will show Texpad as one of the options

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Files imported in this way will be copied to the Texpad-Imported-Files folder in the root of your local iOS storage.