Some uncompressed files or a lot of slides may take up to 10 minutes to export to PDF. We are currently searching how to speed up the process to improve this inconvenience in later releases. 
In the meantime, you have to be patient (and take the opportunity to have a well-deserved break!). 
If the problem is of a different nature, please inform Amanote. We will be happy to improve our software with your valuable feedback.