Found this on HP website:
ISSUE:
When printing to an HP LaserJet printer from Adobe® Acrobat®, Acrobat Approval, or Acrobat Reader for Windows, multiple page PDF print jobs will not collate. Printing similar jobs from other software applications to the HP LaserJet printers collate as expected.
SOLUTION:
This is not an issue with the HP LaserJet printer or printer driver; therefore, there is no solution or workaround provided by HP. Upgrading to Adobe Reader 7.0 or above will resolve this issue; as it supports the collate feature, where earlier versions of Adobe Reader for Windows did not.
CAUSE:
Earlier Adobe applications, such as version 6.x or lower, do not support the collate feature.
Adobe states that Adobe Acrobat, Acrobat Business Tools and Acrobat Reader for Windows cannot collate PDF files, despite the presence of the collate option in the Print Window. This is applicable to most versions of Acrobat, including 4.x, 5.x, and 6.x.