I recently upgraded my Adobe Reader to version 10.1 )Adobe Reader X) and all the buttons disappeared, meaning that you could no longer zoom in or navigate your way around the document; it appeared that the only way to scroll through a document was using thumbnails if the document opened in a browser window or to download the document and open it in a standalone copy of Adobe Reader. Even the annoying buttons which used to appear in Adobe Reader 9 (if you had the mouse in the right place) no longer appeared. These changes rendered Adobe Reader useless for my purposes.
I also tried using right click on an open PDF file to edit the ‘Page Display Preferences’ and clearing ‘Allow documents to hide the menu bar, toolbars and window controls’ under Documents, but with intermittent results.
The solution is to press F8 every time you open a PDF in your browser.