Opening customer provided PDFs can often be a crap shoot, even if the PDF was generated by Adobe Illustrator. There's lots of things the person creating the PDF can do to mess up things.
It's no big deal at all to convert fonts embedded in a PDF to outlines. That's easy. Just place the PDF into a new document, making sure you check "link" when placing the file. Then go the "flatten transparency" dialog box. Make sure to check "convert fonts to outlines" and pull that quality bar on vectors to 100%.
The other stuff involved with opening PDF artwork is the harder part. Lots of people will save .AI files without PDF compatibility to conserve on file size. The same thing goes with creating PDFs from Illustrator artwork. By default the option to preserve Illustrator editing capability is turned on. But you can save a good bit on file size by un-checking that box. Doing so comes at a pretty big cost for anyone trying to import the PDF. Such artwork may feature lots of objects contained in clipping masks or even clipping groups within clipping groups. Some fills and effects may be converted to raster format rather than vector. I'll sometimes resort to that stunt when saving password protected PDFs to give to customers. A rival
sign company wanting to re-purpose that artwork would have some cheese grater on the forehead work to do cleaning up the artwork if they got past the password barrier. Most applications not Adobe-branded that can generate PDFs will default to every trick in the book to minimize file sizes. And that means creating PDFs that can be a real pain to import and edit.
Unless you're working with CorelDRAW X8 or later, Corel is going to be suspect at importing any PDF artwork generated by Adobe Illustrator or really anything else for that matter. Preview the PDF in Adobe Reader or Acrobat DC to get an idea of how the artwork is supposed to look. Then try importing it. Complex fills featuring gradients and levels of transparency are going to be tricky. If the application that generated the PDF made use of certain third party plug-ins that could complicate the situation.
I use Astute Graphics' Vector First Aid plug-in from time to time when importing PDF-based artwork. It's not a perfect solution, but it does cut way down on all the clipping mask objects and clipping groups. There will be less to look at and fix in the hierarchical objects tree within the layers panel.