What happens if you try running:
master..xp_cmdshell 'D:\JPMorgan_IMPORT\pdfcombine.exe D:\431411406.PDF D:\431411408.PDF D:\COMBINED.PDF'
Are any error messages returned? Or is it just the never helpful 'NULL'
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare