How to set up specific file access on the SP site?

General goal: I want to create a library with internal PDF reports. The access to those reports must be personalized and per specific request from users.

Problem: I create the library by adding PDF file. I wanted users have access to the site to see the library Table, searching for the report using metadata in columns and if they find the one to send a request for access. Before the request is approved they shouldn't be able to download or even preview the documents due to confidentiality.

1. However the current options for permission settings in SP do not allow to do it, either they can always navigate the library and preview documents or have no access to the site at all.
2. I tried another option to put all documents in a separate SP site folder and add them into my library as a SP Link (to the file in the separate folder). However when the user click on the Link and do the request it is by default set as request to the whole site (not this specific file). When it is approved, the user is able to preview and download all files in the library.

Could you please advise how can I fix the problems above or suggest other workable solutions to achieve my general goal. Thank you!