Need code to merge PDF files in a folder using adobe acrobat X

[ZVI]

Compile error: User-defined type not defined

As it was said in post #2 and has been mentioned in the comments, you need to set reference to VBE - Tools - References – Acrobat.
Press Alt-F11 and choose menu Tools - References, enable checkbox with Acrobat option (may be also Adobe Acrobat ##.# Type Library, where XX.X is version number), and press OK.