The question goes to what is indexed in a document library and thus returned on the the basic search results page:
a) the Title
b) managed metadata?
c) document content is MS Office or pdf format
d) other?
I am assuming that a search string is entered on a site landing page and thus search is performed in the site and all of its subsets
Managed Metadata absolutely is. You may not be able to refine correctly on it, so you’ll want to create managed properties which are mapped to your Managed Metadata Fields.
How the search chooses to search (site vs site collection) is determined by the search scopes that may or may not have been created in your environment.
Here is a great MSDN post about how search works in SharePoint. https://technet.microsoft.com/en-us/library/jj219669.aspx
