The Installer has a build-in file browser under file menu. But it only shows "The file listing isn't available".
In the installer log:
Installer[82159]: PFPackage::packageWithURL - can't instantiate package: /Users/*/Downloads/Install.pkg
Screenshot:
The problem is the space in the embedded Installer package. Changing "Letter Opener.pkg" to "Letter-Opener.pkg" fixes it.