You cannot use the "Create PDF using Multiple Files" function in Acrobat Pro with pdf's that were created with Adobe Designer (lifecycle). The message is a bit misleading as it's not actually locked. You can check the files security settings in either app but default is "none".
To use that function, create your pdf's in Word, Publisher, etc. Acrobat will convert and combine them from their native format. You can also use image files as well as plain old pdf's created in Acrobat Pro.
Edited by HitSquad, 10 August 2006 - 06:08 PM.