I agree with you that it wouldn't be contributory infringement if the copy existed prior to the purchase. For instance, if I buy a pirated Shrek DVD on the street for $10, I wouldn't be liable for contributory infringement. I can't induce the copy, because the copy has already been made.Maybe it does defy common sense -- but it is consistent with the law. Again, if you can find a case that found a purchaser of a single infringing article liable for contributory or vicarious infringement, I would be interested in reading it. I am unaware of any case where a single purchase, standing alone, was sufficient to impute vicarious or contributory liability for copyright infringement.
But if I give a guy $10 to make me a copy of his Shrek DVD, then it's contributory infringement, because the inducement of $10 is causing him to manufacture the copy.
Here, the OP did not buy an existing copy -- she bought the right to make a copy. The copy did not exist prior to her purchase. After she bought it, the seller sent her a download link. So either the buyer is the direct infringer (by making a copy of the infringing PDF from the link), or if you like to think of the seller as the direct infringer, then the buyer is a contributory infringer.
As you yourself said: "Downloading has been found to create copy, so the act of initiating a download creates a copy, and the downloader is liable for copyright infringement because they made a copy."
If the seller had the PDF on a CD-ROM and sent that copy to the buyer, I agree the situation would be different. But that's not what happened.