I have a document library having documents (eg: word,pdf etc). User has add and edit permission to the library (required) But user should be able to edit a particular document. Can I set the permission as read only for documents for users with edit permission on a library?
Yes. You can set Item level permissions on Document Library. Click the context menu/List Item Menu and click Permissions. Here you can Stop Inheriting Permissions from Library and set unique permissions for the Document.
And once you stop the inheritance of the Document, it will say Item has unique permissions, see below: