If you are using mod_plsql, something as simple as imbedding the file name in an anchor tag will work. You'd need to have apache configured to allow access to the directory your PDF file is in.

procedure show_pdf(
pi_file in varchar2)
is
l_web_root varchar2(200) := 'http://localhost/myroot/pdfs/';
begin
htp.p(' Click for file ');
end;