Hi,

These expressions are often interchanged. However, here are a couple of comments.

1. The applicant should bring the documents as may be required by the authority. 'As' here implies that we don't know at this time what documents are, or may be, required. The 'may be' is often omitted, resulting in a standard phrase 'as required . . .' 

2. The applicant should bring the documents which may be required by the authority. 'Which' implies that we are in a position at this time to know what documents are, or may be, required. It needs to be part of a clause, not just a phrase.

Best wishes, Clive