In PyCharm, how to navigate to the top of the file?

You navigate to the top of the file using Ctrl+Home. It moves cursor too. So does navigating via Page Up and Page Down keys.

Ctrl+Up and Ctrl+Down move the view without moving cursor but scrolling the long file takes some time.

Additionally You can change the keymap (Settings > Keymap). There is 'Scroll to Top' in 'Editor Actions'. You can use Your own key binding for this action, by default (in PyCharm 4 and later) it is not set.

Another way is to navigate to the first line using goto line: Ctrl-G and then 1. But this will move the cursor to the first line.

A small disadvantage is it is a two step process and adds a navigation step. Moving back to your previous location with CtrlAlt-< will have to be done in two steps if you do an edit.