You just have to copy the following code and past into Notepad, then save with .cmd extension and execute it from the path you want to rename.

Rename batch script

rename	"OldName_001.pdf"	"NewName_001.pdf"
rename	"OldName_002.pdf"	"NewName_002.pdf"
rename	"OldName_003.pdf"	"NewName_003.pdf"
rename	"OldName_004.pdf"	"NewName_004.pdf"
rename	"OldName_005.pdf"	"NewName_005.pdf"
rename	"OldName_006.pdf"	"NewName_006.pdf"
rename	"OldName_007.pdf"	"NewName_007.pdf"
rename	"OldName_008.pdf"	"NewName_008.pdf"