@timvandermeij: I've made a number of changes, I really hope that you won't mind too much!
And sorry about the wall of text ;-)

First of all, let me preface this by saying that it's a lot easier to tweak already existing code, compared to writing it from scratch.

This is a (hopefully) fairly complete changelog:

* Added the toolbar buttons that are hidden when the viewer is small to the secondary toolbar. These buttons becomes visible in the secondary toolbar when the viewer is sufficiently small. This uses the new CSS class "visibleSmallView".
I also moved some of the JavaScript code around a bit, since it made this a bit cleaner.

* Places the "openFile" button back in the main toolbar. I'm not sure exactly why it was removed from it, guessing it was by mistake.
Hence there is no need to move the "open_file" l10n strings in all the l10n files ;-)

* Changed the secondary toolbar to have automatic width, hence it will resize according to the content. To prevent it becoming to large, there is also a "max-width" set.

* Set a maximum height for the content in the secondary toolbar, to prevent issues on smaller screens. If it becomes to large, a vertical scrollbar will appear. This currently doesn't look extremely good, but since this is a corner case I think it might be OK.

* Added word-wrap to the buttons, if the text doesn't fit.

* Generally, there was a bit of unnecessary duplication of CSS rules. To be able to fix many of the issues described here, the code needed some re-factoring. Rather than trying to patch this temporarily and then writing a whole bunch of review comments for you to address, I choose to do the refactoring since it was a lot quicker and most importantly allowed me to test all the changes I made.
I *really* hope you're OK with this!

* The CSS rule for the buttons in the secondary toolbar was missing certain styles, which explains why they weren't working correctly. As part of the above refactoring, I based the "secondaryToolbarButton" class on "toolbarButton". With a minimal effort, this led to a number of wins, since the buttons in the secondary toolbar now looks and behaves similarly to all the other buttons in the UI.

* Fixed an issue with the buttons in RTL languages.

* In pdf.js variables and button names should be camelCase. I recently fixed a couple of button id's to use camelCase, but you changed that back in this PR, so I changed it back again ;-)

Things left to do in this PR:

* Fix tab indexes in the secondary toolbar. I removed some of them since there appeared to be duplicates.

* Revert the move of "open_file" in the l10n files.

If you are OK with these changes, I will push them to your branch!