If you have a read only file, that would seem to indicate that the file author doesn't want anyone to make changes
I think you need to ask the author for an unlocked version of the file
PDF files can have usage rights attached to them. There is no "read only" setting, but a number of different rights that the content author gives you - or not. When you bring up the document information in Acrobat or the free Reader, and then go to the "Security" tab, you can see these potential restrictions. Whether you can convert a PDF file to another format is based on the content extraction flag. You could be prevented from changing the document (which would make it "read only"), but if content extraction is allowed, you should still be able to convert the file to a different format. So, if you want to ask the author for a file that you can convert, make sure you ask for one that allows content extraction.