Hi Jeanmichelc,
The Adobe Fill & Sign application for mobile is a standalone application.
It does not have any option to sign-in to the application.
You can simply use it after installing on the device.
Fill & Sign application is automatically synced with the Adobe Acrobat Reader application just to open the PDF form.
It does not sync the data or the signature saved on Document Cloud from the Adobe Acrobat Reader application.
Also, it does not save the changes to the original PDF in the Adobe Acrobat Reader.
Once you fill the PDF form or Add a signature to the PDF in Fill & Sign, it will save the PDF with changes in the application itself.
Then you can either share or send that file to save it to other locations.
Hope the information helps.
Let us know if you have any question.
Regards,
Meenakshi