Hi Denise_Cimarosti
Testing an MYOB Advisor report I was able to successfully create and open that report in a PDF program in my testing.
If you are still encountering that situation I would first recommend seeing if you can open another PDF (possibly a non-MYOB generated one). If you are not able to open that, you would be looking into Adobe itself.
If you are able to open that PDF, the first port of call that I would be doing is clearing your browser's cache and recreating that file AND/OR testing on another machine. It may be seeing if you have multiple businesses that you can create a report for to test if it's just this one business or multiple that you can't produce an openable PDF for.