@lrosenth: one possible workflow is that the users intend to read the PDFs that they open in the application named "Reader." I have never ever one single time in my entire life wanted any of the functionality in any of those panels.
Talking about "proper and correct UI model," it must certainly be obvious that clicking on the button to turn off the panel is non-intuitive. You click the Open button to open a PDF (ok, I never have, but for argument's sake...), you wouldn't then click the Open button again to close the open dialog. You'd click "cancel" in the dialog or some other UI element that implies "click me and I go away."
I am in this thread because despite my substantial experience as a software developer (some of it even building plugins for Adobe products), I had absolutely no idea that turning off that side panel was accomplished via clicking on the thing that turns it on. This is reinforced by the design of the text anyway which implies "tabs", and no tab control ever had you click on the active tab to hide the whole panel.
I don't care if the buttons are there or not, I just didn't want the side panel and there was no obvious way to get rid of it. UI norms and conventions also include being able to disable them from the menu, i.e. the View menu has "Tools," "Fill _Sign," (a rookie programming mistake, btw) and "Comment" options, but the current one is not ticked in the menu (also breaking from UI conventions) and clicking the currently selected one does also not disable the whole thing (which breaks from not only typical/checked UI conventions but also your own internal conventions that clicking on the active thing makes it go away).
So consider that "proper and correct UI" needs to start with consistency, and there is none of that on display here in this particular feature.