Open the image in two ways:
1. Open it AS a PDF OR
2. Open it in it's normal form <<<< That's normally what I do.
So, then you find the Save As button and you click on that, then the image's name/code should be in the typeable box or whatever. Below that is a list of suggested, able, ends to put on it. If it says PDF in that drop down box, then you can save it as a PDF. If it's not there, then it can't do that. I would also save it to the desktop rather than in the same spot.
Good luck!
<3 <3 <3
imawalrus