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.
The simplest approach is to secure the document to just yourself. Then it will only appear in YOUR results and no one else.