Strange tick mark in Preview
Someone else asked this here: https://discussions.apple.com/thread/6754677?start=0&amp;tstart=0

The answer given was "It seem [sic] Preview in OS X has a capability of making a normal PDF (non-editable) editable and I'm guessing that it's doubling up on top of the editable symbol that's already in your editable PDF. I had the same problem and I was making my PDF editable with Adobe Acrobat Pro. I fixed the issue by changing the appearance setting in properties for the checkbox."