Can't open PDF file created in Microsoft PowerPoint in Keynote!?

When i try to open the PDF file by finding the file in 'Finder' and then opening it with a program, keynote is not highlighted as a program that will work. I then tried to force the file to open by selecting 'all applications' instead of 'available applications'. This time keynote is highlighted, but when i try to open the PDF i get a message saying "file could not be handled because Keynote cannot open files in the “Preview Document” format."

Please could someone help as i need to be able to open and edit these PDF files for my degree!!
Many thanks!!