Hi,
You could define just one secondary window type and then
force topics into it as necessary, either by assigning it to the
appropriate TOC entries or by nominating it as the target window
for in-topic Related Topics links (see
this
thread for an example).
Ultimately, I think there are only two ways to open an
external file from a topic in a .chm file:
1) By using a Shortcut control.
2) By using the script described here:
http://msdn.microsoft.com/en-us/library/ms644690(VS.85).aspx
Unlike the Shortcut control, this script displays the
external file in the topic pane of the HTML Help viewer. So, it's
not really ideal for viewing the contents of lengthy PDF documents.
Pete