Is dossier a collective noun of documents?

Collective nouns for documents is a dossier of documents or an archive of documents.

A collective noun is used to refer to a group of people or things. The opposite of a collective noun is a countable noun. If I say there is a flock of birds, you really don't know how many I mean...

Yes, collective nouns would always be common nouns. For example: flock, group, series, collection, herd, shoal. Those are all common nouns.

Collective nouns are words for a group of like nouns. Examples: A flock of sheep. A stable of horses A troop of monkeys A troupe of actors A panel of experts A crowd of onlookers A fleet of cars A...

The word flock is a collective noun, for example a flock of sheep or a flock of robins.