Open
the PDF that you want to serve as the basis of the combined file,
and choose Document > Insert Pages.
Select a PDF that you want to insert into the target
document, and click Select.
In the Insert Pages dialog box, specify where you want
to insert the document (before or after the first, last, or a designated
page of the open PDF), and click OK.
To leave the original PDF intact as a separate file,
choose Save As, and type a new name for the merged PDF.
You can also add an existing PDF with
a currently open PDF by dragging the desktop icon for the PDF you
want to add directly into position in the Pages panel of the open
PDF.