Searchable documents not found linked to knowledge articles. What file types does that support...ie...docx, pdf..etc?
In order for the searchable document search to work there has to be a Full-Text Index Filter installed for that document type. Its possible that you do not have a Full Text Index Filter for the docx file type or pdf.
Attached a MS Word document that has information on how to check which document types are configured in your SQL Server system and also has information on where to obtain the filters for the latest file types used by MS Office (i.e.: docx, xslx) and Adobe (pdf).