Two questions -
1) how to change default option as hand tool rather than select tool?
2) where can i find the setting which sets what the mouse scroll does? Sometimes it scrolls the pages and sometimes it zooms in/ot.
Based on the issue description mentioned above, you want to make hand tool as default as well as want to change the settings for mouse scrolling in Acrobat, is that correct?
Please refer to the following forum threads discussing how to make hand tool as default:
Also, mouse scrolling should scroll down/up the pdf. It will zoom in/zoom out the pdf if using Ctrl when scrolling the mouse.
Try repairing Acrobat using "Repair installation" option under Help menu ( for Windows OS only), reboot the machine after repair and check.
Let us know if you need any help.