MacOS/Gatekeeper

Gatekeeper is a security mechanism built in to Apple’s macOS operating system. It gives computer users the ability to restrict the installation of software based on its origin (using digital signatures).

Installing LibreOffice
In some cases, you’ll need to change the security settings of Gatekeeper before macOS will allow you to install a release build of LibreOffice. After you’ve finished installation, you may change the security to whatever level you like.

UPDATE: Instead of changing Gatekeeper, you may just +click (or right-click) on the file, then choose “Open” to install LibreOffice

macOS 10.9.5
Apple introduced new code-signing protocols in 10.9.5:

"Beginning with macOS version 10.9.5, there will be changes in how macOS recognizes signed apps. Version 1 signatures created with macOS versions prior to Mavericks will no longer be recognized by Gatekeeper and are considered obsolete."

We’ve introduced new code in the 4.4 branch to conform to this new policy, which has become be available by February 2015.

Also see:
 * - Code signing broken by macOS 10.9.5

macOS 10.8 to 10.9.4
Starting with LibreOffice 4.0.3, Mac Intel packages are now signed by The Document Foundation, and should pass 10.8 Gatekeeper without intervention.