Hi xucaen. There is no option for undo/redo buttons on the screen in version 0.6.6, however such a feature has been added in our nightly builds. Nightly builds are development snapshots that contain the latest features and bugfixes, but the are experimental and can also contain new bugs, so keep that in mind while using them. You can download them from Nightly Builds | Pencil2D Animation.
When you start Pencil2D, the canvas should not be zoomed in. If it appears to be zoomed in, it might be because you changed the camera layer without realising, as explained in Clarification on Camera Layer Resolution - #2 by scribblemaniac. Please pay attention to the zoom indicator in the status bar when you start Pencil2D to check if the canvas is actually zoomed in or if it only appears to be due to the camera layer. The way the camera layer works has also been overhauled in the nightly builds, so it should be less confusing compared to version 0.6.6.