Here is a possible solution where Adobe Acrobat does not close after attaching PDF files.
Use of the Acrobat Wrapper:
(Starting with version 8 ) Adobe Reader has changed its behavior when used as a command line tool to print PDF documents. It will no longer close itself after being called with the /t switch. This tool will help you close the Adobe Reader when it has finished printing.
Programs like (WinFax PRO) and a lot of custom built solutions rely on this feature from Adobe Reader.
How does it work?
Most programs that use this feature of Adobe Reader activate it by executing the PrintTo verb on the PDF document they want to print. They do that by calling the ShellExecute API function of the operating system. This tool patches the computers registry database to manipulate the PrintTo verb on PDF documents. Instead of calling the Adobe Reader the verb will activate the wrapper. The wrapper will simply start the original Adobe Reader and close it when it has finished printing.
Rest of details here:
http://www.biopdf.com/acrowrap/close_adobe_reader.php