pdfMerge not working

I have been trying to use pdfMerge but with no luck.
The result is always -3
When I turn on Debug and create a log file, all the file ever contains is
"7/19/2013 5:16:05 PM c:\wpdf\log.txt
Saving: C
Done!"

Any idea what the -3 means and the entry in the log?

Thanks

I do not believe the documents are encrypted ( I can open them in Acrobat or Reader without requiring a password) but I will check for sure.

If I use pdfMergeW (in Delphi) the return code is -10 and this time in the log the file names will appear and the path and file name of the merged document is shown, but still no document is created.

In the first parameter (file number) should this just be the file name(s) of the file or can it also include the path?

Thanks

Having used the pdfMergeW function the log now shows the following

7/19/2013 5:41:03 PM c:\wpdf\log.txt
Loading: C:\WDDC\Internal Applications\WMS Invoice Printing\Temp\CL13005.pdf
Loading: \\RPRTSRVR\pdf_invoices\I6\I1300520130530_IR1517768.PDF
Saving: C:\WDDC\Internal Applications\WMS Invoice Printing\Temp\P07191317410107.pdf
Done!

and the result is -10