Problem with export

Hello,
i have a problem with "export file".
I always export to PDF file for the choir i manage.
But today i try to export to Mpeg 3, and now, when i want to export, it's aways to mpeg 3 even if i select the choice "PDF". Do you know the problem?

I have an other problem : my partition has 21 pages and 11 staffs.
Now when i write a note, it's very very very slow to write and to correct.
My file is about 120 ko.
Do you think it's my computer?
I attach the file.

Regards,
Joseph Matz