Are you having to do further editing to the artwork in the PDF? Is that why you're importing it into CorelDRAW? Otherwise it would seem like an unneccessary step prior to printing. I'll usually view customer provided PDFs in Adobe Acrobat to see how the artwork is supposed to look before I attempt importing it into Adobe Illustrator.
Generally speaking, there are lots of pitfalls with importing artwork from PDF files and it's even more dicey making further edits to a PDF using a vector drawing application. All sorts of unpredictable things can happen when opening/importing the file. I've been using both Adobe Illustrator and CorelDRAW for a long time. Even the latest versions of CorelDRAW can have a difficult time importing PDF or Illustrator AI files accurately. This depends on features and effects baked into the artwork. The file version used also matters. A PDF layout may look one way when viewed in Adobe Acrobat (or the free Acrobat Reader app). Then it looks different when imported into CorelDRAW, sometimes really different.
Some of the same problems can happen when opening PDF files in Illustrator. Like Monika said, a PDF created in Illustrator with Illustrator editing capability preserved should open like any AI file since the PDF has Illustrator AI data appended to it. But a PDF created without Illustrator editing capability preserved or a PDF generated by any other application (such as CorelDRAW) can turn into a mess when opened in Illustrator. There will be lots of clipping masks (and nested masks within masks). I'll also see lots of invisible duplicate paths containing no stroke and no fill.
I often rely on Astute Graphics' Vector First Aid plug-in for Illustrator to help clean-up issues with imported PDF artwork. It can be a huge time saver. The alternative is isolating different objects in the layout on layers and then hunting through the object tree in the Layers palette to deal with unnecessary clipping masks, clipping groups, invisible duplicate objects, unclosed paths, etc. It can be a real time vampire.