The reason is probably that you have not yet published the agenda. An agenda or minutes have to be published in Prepare before you can use the PDF module.
Publish your agenda and choose the agenda or minutes under the PDF module menu. Remember to click on the Create PDF button to start the creation of the PDF.
Finally, the reason could also be that you are not the agenda collector for the committee the agenda lies in. Contact your administrator to be given the rights of agenda collector for the committee.