Beware if you create a signature in Foxit Reader Portable (18.104.22.16896), the signature file will be temporary created and stored in the host computer until the app closes. Signature file path: %AppData%\Foxit Software\Foxit Reader\InstaSign\UserSign\
The entire %AppData%\Foxit Software folder will disappear after closing Foxit Reader Portable. Signature is not transfer to the location of the PortableApp and will not be available on re-opening the portable app.
Windows 10 64-bit Spring Update 2018
PortableApps installed into SD Card