<?php
$inc_file = "pdf/$MLS_ACCT.pdf";
if (file_exists($inc_file)) {
echo '<a href="pdf/$MLS_ACCT.pdf">Click here for Brochure - Open PDF at 100%</a>';}
else {
echo '<a href="property-search-results.php?MLS_ACCT=<?php echo $MLS_ACCT ;?>" target="_top">Further Details >></a>'; }
?>

$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;

echo '<a href="pdf/' . $MLS_ACCT . '.pdf">Click here for Brochure - Open PDF at 100%</a>';