See main post here.
Here I am combining abc.pdf and abc1.pdf to New_File.pdf
Set AcroPDDoc = CreateObject("AcroExch.PDDoc")
Set PDDoc = CreateObject("AcroExch.PDDoc")
'source file from which we will take few pages
b = PDDoc.Open("c:\abc.pdf")
'destination file at the end of which we will insert pages. Although in the last line instead of New_File.pdf you can give abc1.pdf, but I have given a new file name to avoid any confusion.
b = AcroPDDoc.Open("c:\abc1.pdf")
'22: number of pages in the destination file -1
'PDDoc: LPDISPATCH argument for the source document.
'LPDISPATCH is a typedef for a pointer to an IDispatch and IDispatch is the interface that exposes the 'OLE Automation protocol.
'0: The first page of source to be inserted into the destination document.
'1: number of pages to insert
'False: Whether Bookmarks are to be copied or not.
b = AcroPDDoc.InsertPages(22, PDDoc, 0, 1, False)
'saving combined pages to new file, you can choose the destination file also.
b = AcroPDDoc.Save(1, "c:\New_File.pdf")