I warn people who only use CorelDRAW to keep Adobe Reader handy to proof those client supplied PDFs and compare what they see in Adobe Reader to the imported results in CorelDRAW. Things often go very wonky in unexpected ways. I've run into trouble opening client supplied PDFs in Adobe Illustrator as well. Many applications can generate PDFs. There's lots of different settings. Un-checking "Preserve Illustrator Editing Capabilities" when using Illustrator to save a PDF can introduce all sorts of issues. Astute Grahpics' Vector First Aid plug-in can be a tremendous help. It can fix a lot of problems in things like messed up PDF files instantly, saving potentially hours of tedious work.