Acrobat - batch convert using Applescript - ..."Adobe Acrobat Pro got an error: document 1 doesn't understand the save message."

Hi, hoping someone can help with this...just at my wits end with this.
 
I have a series of .png files that represent individual pages of a group of multi-page files (in this case, sheet music). Each file is numbered sequentially, e.g. first_song01.png, first_song02.png, first_song03.png, second_song01.png, and so on). I have written as script to open all the files of each song's .png files in Acrobat Pro 9 for Mac OS 10.6 at a time.
 
That automatically causes Acrobat to ask if I want to group all these similarly numbered files in a single document. I've got it UI-scripted to click "yes," and Acrobat opens the files into one document.
 
Next, I see a dialogue asking me to name the file. No problems up until this point.
 
Now, it falls apart. I have been trying to get Acrobat to save this newly created document to a particular folder (specified earlier in the script during runtime) with the filename provided in the previous step.
 
Script looks like this:
 
tell application "Adobe Acrobat Pro"
tell document 1
set theNewFileName to (text returned of ((display dialog "Enter the File Name." default answer theSuffix with title "Enter File Name" buttons {"Cancel", "OK"} default button "OK" cancel button "Cancel" with icon note)) & ".pdf") as string
log "theNewFileName: " & theNewFileName
set theNewFilePath to targetFolder & theNewFileName
log "theNewFilePath: " & theNewFilePath
end tell
end tell
  
tell application "Adobe Acrobat Pro"
save front document to alias theNewFilePath
close front document saving no
end tell
 
I have been fiddling and fiddling with this, to no avail. I keep getting the same error:
 
Adobe Acrobat Pro got an error: document 1 doesn't understand the save message." number-1708 from document 1
 
I assumed this had something to do with the alias being properly defined, but I've included a log entry just above this code where it confirms that points to something like the following:
 
log theNewFilePath: Hard Drive:Exports:Scans:first_song.pdf
 
Following that, the Applescript editor shows the result of the attempted save command:
 
save document 1 to alias {alias "Hard Drive:Exports:Scans:", "first_song.pdf"}
error number -1708
 
Any idea what I'm missing here? This is making me nuts and I'm really out of ideas.
 
Thanks for help in advance,
Matt