The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From Viafirma S.L:
Viafirma inbox allows viewing, electronic signature, validation and custody of all types of electronic documents.It is an "Electronic Signatures Agenda" which make easier the pending-of-signed document management providing them an approval and signing workflow.This native app for iOS follows the pattern of e-mail client organizing the signature agenda thanks to the notification and planning of the documents expiration date in signing.