Open File - Security Warning - How to Disable

Hi, I know others have asked similar questions, but I'm not sure those answers apply to my situation.

Something must have changed after a recent Windows update, because over the last week or so, when I try to open a downloaded PDF (downloaded from my Chrome browser or from YouSendIt to my desktop), I'm getting the dreaded "Open File - Security Warning" message.

It reads:
Do you want to open this file?
Name: xxx.pdf
Publisher: Unknown Publisher
Type: PDF file
From: C:\Users\MyUserName\AppData\Local\Temp\XXX

This message window does NOT give me an option to uncheck a box to "always ask when opening this file type."

How can I disable this message? Thanks so very much.

Thank you. I feel pretty silly now. It worked once. I hope it works for all my cases.

I used the batch file to disable it for all users but it doesn't work

edit:
Disable "Open File - Security Warning" Message - door2windows
work great