Pronouns which are used as an adjective are demonstrative pronouns. For instance- this, that, these etc.
The difference between the two is that demonstrative pronouns replace the noun phrase and demonstrative adjective modifies the noun phrase.
Therefore, a sentence which contains a pronoun used as an adjective is I chose that bird for its bright coloring.
Hence, here 'that' is a pronoun which acts as an adjective demonstrating the bird of which it is talking about.
