function IsPDFFile(aFileName: TFileName): Boolean;
var
Buf: array[0..3] of byte;
Stream: TFileStream;
begin
with TFileStream.Create(aFileName, fmOpenRead) do
try
Read(Buf, SizeOf(Buf));
Result:= (Buf[0]=$25) and (Buf[1]=$50) and (Buf[2]=$44) and (Buf[3]=$46);
finally
Free;
end;
end;