What is the best and easy way to restrict a one document in a document library not to show in the SharePoint 2013 results page
If it is just one document in a document library of many files, I would go into Central Admin and create a Search Exclusion rule for the file and it won't appear in the index. It might be listed as Remove Search Results, I only have 2010 on prem atm and 2013 delivered via Office 365 so I can't verify the exact language in Central Admin of 2013 on prem.
An ootb approach would be to enable content types on your document library and utilize the Link to a document content type. You would store the document you don't want to appear in search results in a separate document library that is excluded from search. You would utilize the link to a document content type to create a link to this file in the document library that has all your other files.