Ehm. Before placing? At that point ID doesn't know anything at all about the PDF.
var myPDFFile = File.openDialog("Choose a PDF File");
Then check the lower left bottom -- it should display the page size.
Why not first place the file and then check its size? You can always move it to the right position after placing.
Oh, by the way: "a PDF" doesn't have a "page size". Each single page in a PDF can have a page size of its own.
befor placing becuse befor placing user known the real PDF page size and can be change it to place