You can do that using concatenation and ternary operator <cond> ? <if-true> : <if-false>
:
$reports_list .= "<tr>...<td>"
. (file_exists('reports/$id.xlsm')
? "<a href='reports/$id.xlsm' download>Download Attached File</a>" : "")
. (file_exists('reports/$id.xlsx')
? "<a href='reports/$id.xlsx' download>Download Attached File</a>" : "")
. "</td></tr>";
It is up to you to decide whether it is more or less readable than just using several if statements -- this way is suggested more for shorter and simpler conditions.