Is there a way where I can convert a pdf file from a url using pdf service api?
Have you tried using one of our SDK sample code projects to test with? There is no reason why you can't use a document that's stored on a server somewhere.
FileRef.CreateFromURI() method only works for HTML to PDF operations as of now.
However I managed to download the stream and use FileRef.CreateFromStream() with one caveat. It couldn't worked when downloaded stream using HTTPClient framework class and worked only with WebClient class.