Thank you very much again, Tomos, that was dumb of me! (I just checked that they were yours, without taking any further interest in them, or really read them to begin with!)
In fact, deselecting the two settings solves my problems partly, I now have the menus displayed in every case (I hope, and except when the pdfs are displayed directly in my browser, I fear, meaning before downloading them in order to just be able to read them properly!!!), and it's always in maximized window, so that's a real step forward, thank you very much again!
But I'm far from being really happy with Adobe Viewer, and from my further tries (I tried before, too), it doesn't seem I can do anything about these problems here, hence my continued search for a better alternative:
In the Settings - View - Page Display, you can NOT disable "enable scrolling" - you only can set this for any given document, one by one, when viewing it, and every new document disables your previous try to disable this, then. This is VERY BAD! In fact, this "scrolling" means whenever you press pgdn, you find yourself at any unwanted position of the page or next page, but the (wanted) disable option makes behave the pgdn key work this much better way:
It goes to top of next page; to the rest of that page (if it wasn't displayed in full previously); to top of next page; as before; as before. In practice, this means, you can browse your pdfs page-wise, by two such key pressings, or even, by setting the zoom accordingly (and whenever the original font isn't too small), by one key pressing (one key pressing, one new page, this is ideal (ok, it would be perfect if it was the rightarrow key, not the pgdn key) and of course should be the dafault and not only be available by resetting this in the depths of the menu for every new pdf again and again); the default "allow scrolling", on the other hand, means you never know where to read on after pressing pgdn, since there is always some overlapping, and not even by a fixed amount of pixels or text lines; this greatly interferes with normal reading.
And there is another problem, it does NOT seem to be possible to have, by default, pdfs displayed in "full height", meaning to have Adobe Viewer set the zoom in a way that the height of one page just fills the height of the maximized window (as before, I'm not speaking of "full screen", meaning without menus, here).
In practice, this would mean that you could read most pdfs "full page" since the font of most pdfs is so big that its original size is much too big for the screen, and so big that it's perfectly possible to have it resized to a bit less of 1024 pixels per page (maximized window less the caption and the start bar below), AND it could be read without problems, and if not, in exceptional cases, there would always be the poossibility to make the zoom bigger.
But it's even in the help file: "Default Layout And Zoom
Page LayoutSets the page layout used for scrolling when you first open a document. The default setting is Automatic. The Page Layout setting in File > Properties > Initial View overrides this value.
ZoomSets the magnification level for PDF documents when they are first opened. The default setting is Automatic. The Magnification setting in File > Properties > Initial View overrides this value.
Note: Two conditions can affect page layout and zoom. 1) Someone has already set an individual PDF to a different initial view in File > Properties. 2) You have the option Restore Last View Settings When Reopening Documents selected in Edit > Preferences > Document category." (Number 2 here would not be of any help, since it just means reopening of the SAME document, and within the SAME session.)
And this means in practice that for almost every pdf, I have to fiddle around either with the zoom, or with the "Allow scrolling" setting, or, in most cases, with both, and that's really awful, it's just that I now can do without the "maximize window", first of all.
So Adobe Viewer is really a very bad thing (or did I overlook some setting, again? but I fear not), so a better alternative would be more than welcome.
Of course, this leaves the question why most people creating pdfs do not see these problems on the "consumer" side when they create them, with so bad settings (then overriding anything you could set in Adobe viewer).
Again, the way Adobe viewer does it (in version X, so what about 11?), means you have to fiddle around with EVERY document, for zoom, and for scrolling setting, when with a viewer where your settings would not be overridden by "individual" document settings, you would have to fiddle around with just SOME documents. (Your experience with regards to font sizes may vary if you have lots of scientific pdfs, all with rather tiny fonts, but for most "consumer" pdfs, the problem is as described above, and the scrolling setting problem persists in any case.)