Searching in SharePoint is a quick way to locate content that you don’t necessarily know where it resides. Search is navigationally driven by where you are searching. Below are guides based on how search works with navigation.
If you know that the content is located within a library, it is best to go to the library to search for the document. As search will only look for the item within that library.
However, if you do not know where the document is, but you know what site it is located on, you can browse to the site and search from there. This will bring back results from the entire site. So you may find more results related to your search because it spans the entire site and not just a library:
Because of this, my search result was at the bottom of the 3rd page. However, if I searched for the file name and the author, there was only one item that matched that search: