expired domains backlinks retaining value

I found expired subdomain with some good backlinks. But most of these backlinks are pointing to pdf and doc files.

what should I do? I mean what is better? Because I red search engines may reset backlinks power if site is not topic relevant anymore


1) restore needed .doc and .pdf files from wayback machine

or

2) don't restore anything and use .htaccess to 301 redirect everything to single new html page with one good topic relevant article

or

3) do 2) and create HTML page based on content from .doc and .pdf files

You could restore the URL's that are getting most of the backlinks and add content related to what's in the .doc and .pdf if you have it. Then create a niche relevant content section and drop a link back to your homepage. That is if you plan to link out from the homepage.

Thanks ill try some of what was said in replies

You should keep the niche with your new website but depending on the CMS of your website you could find a plugin or module to redirect 404s to your root domain. I know wordpress has such plugins and they will move some of the authority from the lost backlinks to the root domain

There are a few type of 301 redirects - the ones that retain the full link value, ie mypage .com/first-page.html, which will redirect to newpage .com/first-page.html, and the one that redirects everything to root domain.
I think you should create a redirect, that redirects everything including whatever subdomain to your root domain, that way, the link juice will flow to your root domain, and nothing is left behind - make sure and double/triple check to ensure every subdomains is redirected to root.