Thank you for the feedback.
Basically, you need to use HEScript to open external PDF files.
In ExeOutput, Application Settings => Scripting
Double click on UserMain and add this:
procedure OpenSubFolder(PDFName: String);
var
EbookPath, MyFolder: String;
begin
EbookPath := GetGlobalVar("HEPublicationPath", "");
MyFolder := EbookPath + "\SubFolder" + PDFName;
OpenFile(MyFolder, "", SW_SHOWNORMAL);
End;
Replace SubFolder with the name of your subfolder with PDF files.
Then in your HTML code, change to
More info about this syntax at http://www.exeoutput.com/help/callscript.htm