Is the word beautiful a pronoun?

No, the Word 'beautiful' is an adjective, a Word used to describe a noun.

Examples:

It is a beautiful day.

Mother made a beautiful birthday cake.

Beautiful flowers adorned each table.

The dress looks beautiful on you. (predicate adjective, follows a linking verb to describe the subject noun 'dress')

A pronoun is a Word that takes the place of a noun in a sentence.

Example:

The dress looks beautiful on you. It is a good color for you. (the pronoun 'it' takes the place of the noun 'dress' in the second sentence)