Your code that checks whether the file exists looks good. Here's a rundown of how PHP parses variables in strings:
There are several ways to use variables in strings:
$name = "Jeff";
// parsing in double quotes
$string = "My name is $name";
// you can also use curly braces for better readability
$string = "My name is {$name}";
// or you can concatenate
$string = "My name is " . $name;
In single quotes, only the concatenation method works. You should do something like this:
echo '<a href="pdf/' . $MLS_ACCT . '.pdf">Click here for Brochure - Open PDF at 100%</a>';
and you should have the variable printing correctly.
Finally, in your last echo statement, you have <?php ?>
in your string. That will print literally - you only use those when you're switching to PHP from pure HTML. Instead, use the same string concatenation method described above.