PHP question: how to combine syntax?

This part is incorrect:

if ( is_single() && !  'wpdmpro' == get_post_type() )

That essentially means:

if ( is_single() && false == get_post_type() ) {

Which will never be true. To check if a value does not equal another value, use !==:

if ( is_single() && 'wpdmpro' !== get_post_type() )

Also, the way you’ve combined the two if statements is hard to read and prone to errors. Instead, just add the first statement as a condition to the one statement:

if ( function_exists('readingtime') && is_single() && 'wpdmpro' !== get_post_type() ) {      
    echo '<span class="text-sep text-sep-cat">/</span>';               
    echo readingtime();
}