Change the file name:
l_file := UTL_FILE.fopen('BLOBS','MyPDF.PDF','wb', 32767);
If you have the filename stored in the table you can just use it:
create or replace ...
is
l_name varchar2(255)
...
SELECT report_name, REPORT
INTO l_name, l_blob
FROM myReportstable
WHERE someIDcolumn = inKey;
...
l_file := UTL_FILE.fopen('BLOBS',l_name,'wb', 32767);
...