Greko1,
A new image box should have two variables: "Page Number" and "Image 1". Image 1 is where you specify the pdf's name, but Page Number is where you need to put the page that you want to display. I'm thinking that you probably put your expression in the Image 1 variable, and so it's looking for a file called "20000", which doesn't exists.
That's one thing. The other thing is that PrintShop Mail doesn't have a real way of looping through the pages of a variable pdf file, so if you say "20000" in the page number, it will show page 20000, it will not display the pages from 1 to 20000.
However, here's a trick. In your Layout's properties, you should find "Number of Copies", and you'll be able to use a variable expression in it. Use TOTALPAGES("54127_QRCodes.pdf") for the Number of Copies, as we want one page per pages of the pdf. Next, in the image box's "Page Number" variable, use PAGE_NR() as the expression. PAGE_NR() returns the number of the current physical page, which in this case will match the pdf's page count, since we use TOTALPAGES for the Number of Copies. This will allow you to simulate a loop through all the pages of the pdf. Note that if you have more than one record in your databage, you will need to split the job every records, because PAGE_NR() will not reset from record to record, it counts every physical pages.
Hope that helps!
Regards,
Raphaƫl lalonde Lefebvre