You set the pdf in step 3, inside the pdf object, the uploading is done later in step5

If you wish to first upload and then set is as link from internet, then you need to contact your website in step5, use the upload button to upload to a folder or root.

I assume you name the folder mypdf  and the pdf file itself is book.pdf

the pdf link will will be yourwebsite.com/mypdf/book.pdf. (if you use no folder the foldername can be skipped)

now you can link in the pdf object to this html adress.

Both methodes should work. I don't know why you get the error.