If you are required to sign a PDF document, an active regular BankID will be required. You can not get BankID on mobile without having regular BankID, but the challenge is that some have "forgotten" that they have the usual BankID and thus the password that can be used to sign a document.
Common BankID and BankID on mobile are essentially two different systems, where only BankID currently supports the signing of PDF documents, while BankID on mobile only supports the signing of short texts, as well as login. To sign a PDF document, you have two options:
1) use OTP generator / otp app on mobile / otp sheet
2) Use BankID on mobile as a single-tag mechanism in regular BankID.
See screenshots below that show you how to choose BankID on mobile as a one-time password inside regular BankID.
If you do not get this menu, either regular BankID expired or one-time mobile phone is not supported by the customer's bank. Then, please contact his bank to clarify this. (BankID may have expired even if BankID on mobile works)
See also this article from DNB explaining why you can not "throw the otp generator" yet: (In Norwegian) https://dnbfeed.no/privatokonomi/200-000-roter-bort-kodebrikken/