the file doesnt exist on the server.
In the download behavior, yo uhave set the Folder to:
and the file name is coming from the hiddenField form element.
for the third button, the value of the hidden file is "VA_K4_13_Bannerplakat_CC Vest_800x650.pdf":
<input name="hiddenField" type="hidden" id="hiddenField" value="VA_K4_13_Bannerplakat_CC Vest_800x650.pdf">
so it is looking for the following file:
this URL goes to a 404 error page. The file does not exist so cannot be downloaded.