I tried this Java Step in an Activity (it's a mangled-version of the Code-Pega-PDF.VIEW activity) on a PRPC6210 system (which I happened to have running  - it should still work on later versions of PRPC).

ClipboardPage objHeadersPage = tools.findPage("pxRequestor").getProperty(".pyHTTPResponseHeaders").getPageValue();
objHeadersPage.putString("contentType" ,"text/plain");
objHeadersPage.putString("ContentDisposition" ,"attachment;filename=test.txt");

String contents="Hello World";
byte[] bContents=contents.getBytes(java.nio.charset.Charset.forName("UTF-8"));
String result=tools.sendFile(bContents,"test.txt", false, null, true);

This works when I launch it from Designer Studio - I haven't tried it in other PRPC contexts (I don't forsee any problems there though).

This launches Notepad on my system; but it is actually leaving the decision to the Browser/Client Operating system to find a 'matching' program for 'txt' files.

The 'Content-Disposition' header hints to the browser that this should be treated as an attachment, rather than (the default for 'text/plain') launching 'in-browser'.

On my system ; the browser behaviour is to ask the end-user what they want to do with the file:

This is a 'one-way' process; the resultant text editor (notepad in my case, and in most default cases I guess) is not in any way able to save the data back to the PRPC.

Cheers

John