These files are frequently password-protected. If the "post" you were looking for was meant to provide a password, common defaults often include the name of the website/channel where you found it (e.g., @channelname ).
"DDN" is also an acronym for Direct Data Network or Data Distribution Network , which might imply the file contains datasets or configuration files for network software. A1 DDN.7z
Once installed, right-click the file and select "7-Zip > Extract Here" or "Extract to A1 DDN/" . These files are frequently password-protected