I played your game twice. As far as spoiler free feedback goes, the biggest change to the game I'd like to see is to have the player click to finish reading the notes. On my first time through I did miss part of one of the notes because it advanced when I wasn't ready. On my second run I wanted to be able to advance through the identical stuff faster. Not being able to quicken the pace was the biggest deterrent to playing again to see new endings. I think the atmosphere of the game was effective. I thought the music worked very well. I would have personally preferred the main voice to be just a little louder.
I think comprehensive feedback of this game requires spoilers, so this is the warning to anybody else who might want to play the game first.
The other people that come up behind you are probably my favorite part of the game, yet also where I see the most room for improvement. The voices for them - which I'm guessing were computer generated - really ride the line between creepy and comical. I can definitely see some people getting taken out of the atmosphere by them. That said, I got past it fine myself.
It's kind of hard to give feedback on a story as abstract as this. I guess I'll just tell you some thoughts I had and you can decide for yourself what to make of them. The first ending I got was the window ending. I wasn't sure what about my choices led me to that ending, as it seemed like the notes were telling me that I was already set on that course, and I thought I was choosing to change my course. Maybe it's because decided not to insist on opening the door was considered giving up? Or maybe it's because the person giving the notes was untrustworthy? The beautiful ending was my second one, which made more sense to me. I interpreted it as a wistful "if only" type of ending. Not quite as dismal as my first ending, but still not good. Just my character imagining what could have been I suppose. Based on these two endings, I'd interpret this as a story about sticking to your guns, trusting your instincts, and seeing a task through to its end.
I hope this feedback is helpful to you. This game seems pretty complete, so I'm assuming you don't have any intention of updating it further. If that is the case, I hope some of this helps you on your future projects.