When viewing PDF file or image using Preview on the Mac, zooming in (⌘+) and out (⌘-) often is inevitable when you try to adjust for the optimal viewing size. If you need a close-up look, hit the ` key (the left most key on the numeric row, above the Tab key) to use the build-in magnifier tool. Alternatively use the menu Tools > Show Magnifier.
The magnifier is movable with the mouse for you to target the viewing area. Hit the Esc key to dismiss the magnifier tool.