I am doing a security assessment where a webpage has fields if we sbmit data in them pdf is generated at backend, is it possible to submit some malicious text that will come in pdf and can compromise the user or backend machine.Any input is most welcome.
No, this is not possible as far as I know. I cannot think of a vector that would allow that to happen.
However, it is a rule that ALL user input is considered untrusted and must be validated before use. That still applies here. A classic example is allowing unrestricted length inputs (a non-browser client can ignore any HTML "restrictions" on length) to be submitted that causes a buffer overflow on the web server.
So always sanitise and validate any input to the server before you use it.