You can use:
<a href="yourscript.php?variable1=one&variable2=two&variable3=three">Link</a>
And then in php:
$variable1 = $_GET['variable1']; // one
$variable2 = $_GET['variable2']; // two
$variable3 = $_GET['variable3']; // three
In your example:
<a href='invoice_details.php?ppid=".$row['sno']."&pinv=".$row['invno']."'>link</a>
Here is a little explanation about & notation: Encoding issue, coverting & to & for html using php
It is better to write html links with entity & then simple &.
More docs on get: http://php.net/manual/en/reserved.variables.get.php.
Remember to run some check on your get values, and not save / post them directly on site as this can make some security issues.