The below link explains how to download ADK 8.1 files for offline use. The link explains both GUI and command line methods.
About Nirmal Sharma
Nirmal is a MCSEx3, MCITP and was awarded Microsoft MVP award in Directory Services. He is specialized in Directory Services, Microsoft Clustering, Hyper-V, SQL and Exchange and has been involved in Microsoft Technologies since 1994. In his spare time, he offers technical help by writing tips and articles. Nirmal can be reached at email@example.com.