The dangerous bend or caution symbol ☡ [...] was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.
Please, stop recommending Adobe Reader. Or even worse, claiming that your clients need it to open your email invoices. Because what could go wrong?