When you add watermarks to a PDF document, you want them to be visible unobtrusively. The following settings for the Watermark tool in Acrobat (Document > Watermark > Add) work best for me:
Font: Arial
Size: Varies. For shorter watermarks like the word Draft, 72 pt works well.
Color: Pure red (R: 255, G: 0, B:0)
Rotation: 45°
Opacity: One notch above zero on the slider (12%)
Location: Appear on top of page
If required, you can specify different watermarks for different sets of pages. Click Page range options in the Add Watermark dialog box to do this.