Hi sat,
Place your button inside div and set the position to fixed, bottom and left to 0px.
Check the below example.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
</head>
<body>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" />
<div style="position: fixed; bottom: 0px; left: 0px;">
<input type="button" value="Save" class="btn btn-danger" />
</div>
</body>
</html>
Demo