#refs a[href$=".pdf"] {
  display: inline-block;
  padding: 2px 8px;
  border: 1px solid currentColor;
  border-radius: 8px;
  text-decoration: none;
  font-weight: 600;
  margin-left: 6px;      /* a little space after the citation */
}
#refs a[href$=".pdf"]:hover { text-decoration: underline; }