Adobe Reader XI provides several ways to adjust the PDF's page zoom.
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--| Ctrl+Y opens the "Zoom To" dialog.
--| Ctrl + (the "+" key) zooms in.
--| Ctrl + (the "-" key) zooms out.
--| If using a scroll wheel mouse Ctrl + roll the wheel (zoom in or out).
--| Use the "-" or "+" on the toolbar to change zoom.
--| Use the zoom percent drop-down to select a zoom setting.
--| Set Adobe Reader's default application zoom
(note: A PDF having its Initial View configured to a specified zoom will override the application default.)
----| Select Edit > Preferences > Page Display
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Be well...