I am using a Macbook Pro, operating OS X Yosemite 10.10.2
When I started the app, it suggested I upgrade to Version 2.6 (25480), which I did, before starting the tutorial.
I was following the tutorial, Part 1, Step 3, and clicked on the pdf file spacewalk_info in the Research folder. I scrolled down a few pages of the folder and was then unable to return to the tutorial, either by clicking the back arrows, clicking on the file name or using the up/down arrows. I closed the app, restarted it to no avail. I tried deleting the file; it showed as deleted in the binder window, but the file remained in the editor window. [I undeleted it after that]
I tried closing the file from the file menu, but when I reopened, it went back to the same pdf file page. Thereafter clicking the Close from the file menu has no effect until I restart the app. Using the Close project behaves the same way.
Closing the file and opening the tutorial again just returns me to the pdf file.