I think the problem may be that at the end of each of your html files, there is a </body></html> - and then when the next one starts up, there will be a <html><head> - which, I imagine, causes problems.
One way around this, that I can think of, is to turn all the html pages into php. Then turn the content of those pages (ie <body....</body>) into a function:
//functions.php
function ab() {
//switch to html
?>
<body>
content of page here
</body>
//switch back to php to close function
<?
}
?>
This also means you can put all the content of those 6 pages into 6 different functions within a single file. Don't forget they wont have a head, or a footer, though, so maybe at the bottom of functions.php do 2 more functions:
function PrintHead($title) {
print "<html><head><title>$title</title></head>";
print "<body>";
}
function PrintFoot() {
print "</body></html>";
}
Then you can just call up the different "pages" by doing:
include("functions.php");
PrintHead();
ab(); //prints the body of the first page
bc(); //prints the body of the second page
// etc, for as many as you need...
PrintFoot();
Hope that helps...