Is across a pronoun

No, the Word 'across' is a preposition and an adverb.

• A preposition is a Word that connects a noun or a pronoun (the object of the preposition) to another Word in the sentence.
• An adverb is a Word that modifies a verb, an adjective, or another adverb.

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

Examples:

There's a drug store across the street. (preposition, connects the noun 'street' to the noun 'drug store')

I came across it while looking for something else. (preposition, connects the pronoun 'it' to the verb 'came')

He came across as a friendly fellow. (adverb, modifies the verb 'came')

I came across it while looking for something else. (the pronoun 'I' takes the pace of the noun for the person speaking; the pronoun 'it' takes the place of a noun for a thing previously spoken about)

He came across as a friendly fellow. (the pronoun'he' takes the place of a noun for a male previously spoken about)