A PDF can contain just about anything, so while Illustrator will read the file and interpret the contents, there are no guarantees as to what you can do with it from there. It may be a raster image (made up of pixels, like a photograph; Illustrator can't edit that), or, if there is vector data that is not Illustrator-native, it will likely be contained within one or more clipping masks. You can release those clipping masks (Object > Clipping Mask > Release), to make their contents accessible, however, it's possible there are multiple levels of masks nested within each other, and it's also possible they contain hidden objects that you wouldn't want un-hidden, potentially many of them...and some or all of those could be raster images.
Because it's simply not possible to predict the result of opening a PDF (that didn't originate in Illustrator, or wasn't purposely saved with 'compatibility' options enabled), it's often repeated here that "Illustrator is not a general-purpose PDF editor".