Could you please let me know what version of Acrobat/Reader are you using.
Was this PDF created using Acrobat or any other tool?
These lines might occur due to flattening of transparency in that file.
Try zooming in or out as you will most likely see that these hairlines will disappear.
Also, checking 'Enhance thin lines' and 'Smooth line art' options under Edit> Preferences> Page Display
Let me know if this helps.