I see the usage of both 'User Manual' and 'User's Manual' in daily life. I am wondering if both of them are grammatical and idiomatic? If 'User Manual' is grammatical, is 'user' used as an adjective here (rather than possessive pronoun)?
In the phrase user manual, the word user can be described as an attributive noun. The phrase is both grammatical and idiomatic.
User's manual is also both grammatical and idiomatic. Feel free to use either phrase.