Here's a basic Activity I put together to show the principal of this - you will need to put in checks to ensure the OBJ-OPEN doesn't fail etc.

First here's the steps, params and Pages (I haven't expanded the steps here - see below for this).

Here's it running:

So here's the first two steps expanded:

The third step doesn't need expanding (nothing extra to show); and here's the Java Code from the Java Step.

(Note : see Code-Pega-PDF.View - which does a similar thing).

com.pega.pegarules.pub.util.Base64Util decoder=new com.pega.pegarules.pub.util.Base64Util();
byte[] byteArray = decoder.decodeToByteArray( filesourceb64 );
String result=tools.sendFile(byteArray,filename,false,null,true);

Hope this helps, Cheers John