Pin any document(s) to the top of your Documents page by right click or using three dots menu on a document, then choose Pin under Additional actions.
Follow same steps to unpin a document.
Pinned documents will show in a green highlighted row at the top of the first page irrespective of any column sorting applied.
In Folder view, pinned documents will show at the top of the page above any folders, with folder contents showing underneath.
Using Search or Filter will also apply to pinned documents, and they will not show if search/filter conditions are not met.