div.a4 {
 border : 1px solid #000000;
 width:595.3pt;
 height:841.9pt;
 margin:18.0pt 19.3pt 18.0pt 18.0pt;
 mso-header-margin:35.4pt;
 mso-footer-margin:35.4pt;
 mso-paper-source:0;
}
.a4 .footer{
 background-color:#C0C0C0;
 height: 50px;
 bottom : 50px;
 margin-left: auto;
 margin-right: auto;
 }

<div class="a4">
  1
  <div class="footer">12345</div>
</div>
<div class="a4">
  2
  <div class="footer">12345</div>
</div>

bottom : 50px;

margin-top:790pt;