Like Microsoft Word, Acrobat likes to create a new window instance and taskbar entry for every document opened. This is no big deal while reading a single PDF, but if you typically have several open, as I do, the accumulation of all those separate taskbar buttons can be annoying and counter-productive. Afterall, taskbar space is limited, and a power user may have numerous applications vying for space; if documents within those applications generate their own taskbar buttons, it leaves even less space.
If, also like me, you don’t want every PDF to create a new taskbar button, you can revert Acrobat to standard MDI (Multiple Document Interface) behavior.
Open the Acrobat preferences from Edit > Preferences. The last option on the General pane’s Miscellaneous section is “Show Documents in Taskbar (Takes Effect After Next Launch of Acrobat)”. Uncheck that and, after closing and relaunching Acrobat, only a single taskbar entry will ever appear regardless of the number of PDFs opened.
To navigate between opened PDFs when Acrobat is set to standard MDI mode, use the Window menu, which lists all opened PDFs by their titles, or use the CTRL+F6 and SHIFT+CTRL+F6 keyboard shortcuts to cycle between opened documents forward and backward, respectively.