Sometimes, but not always, you can open the pdf in Acrobat Reader and then re-save it with a different name. I've sometimes been able to open a persnickity pdf in CorelDraw that way.
If you have another vector drawing program, for instance Xara, then it may be possible to import it into that program and then export it out as a cmx file ( Corel Presentation Exchange format ) and then import that into CorelDraw.
I work in the sign business and I'm getting pretty good at working with "iffy" file. There are times though that a file just won't open or it brings the whole show to a standstill. When that happens I'll usually talk to the designer at the other end and see if they know womeone close by with a PC that can open the file and resave it as something else.
I find that sometimes I have to be more creative in getting a file imported than I would if I have to come up with a concept drawing from scratch. One thing I will never do though is buy Adobe Illustrator just for opening files. That represents a stack of beer money.