There is a Check Spelling option under the Edit menu, but it's hard to use. What you could possibly do is go to "File" and "Save as Other" and save it as a Word document. If the text isn't flattened already, it will show up in the Word doc. You can run your spell check as you normally would and then mark the changes on your PDF version. (If the text is flattened, it's basically like a picture, and spell check won't work in any program.)