Append PDF is not supported on any version of Go.
The Temporary Path and Documents Path are both valid paths for FMGo.
Sample Script :
Set Variable [$Output; Value: Get( TemporaryPath ) & "sample.pdf"]
Save Records as PDF [No dialog; "$Output"; Records being browsed]
To save to Documents Path
Set Variable [$Output; Value: Get( DocumentsPath ) & "sample.pdf"]
Set Variable [$Output; Value: "sample.pdf"] //FMGo defaults to the documents path
You can put PDFs in container and use FM Server to merge them.
e.g. by using a plugin function on the server script to do this.