Comment 1 for bug 333953

I'm setting this as invalid. Here is the rationale: this is basically already implemented but you're using it wrong. If you import a PDF (or any other file) into an already created document, it won't change any attribute of it like page size, etc. Instead, if you open it, it will take the page size from the PDF file.
So if what you want is to get the dimensions of the PDF page, open it instead of importing it.
If what you want is to import a PDF into an already created document, create a template and import the PDF. This way you'll have some basic settings coming from your template and the drawing of your PDF.

Feel free to revert the status if you disagree.