[plugin: Voucherpress] Cannot open pdf voucher

Hi
Having a bit of trouble with Voucherpress. I installed the plugin and create a voucher but when I “preview” or “save”, I save it alright on my desktop but when I go to open it, I get this message:
“adobe reader could not open the file because it is either not a supported file type or because the file has been damaged”

Any ideas on what I’m doing wrong?
Thanks