Hi there Fiona, thanks for reaching out!
When exporting documents as a PDF, only external links or links to email will remain active. With all other forms of exporting and publishing links will not function.
Regarding the collapsible sections of your mind map working after the export: Unfortunately, this is not currently supported in Lucidchart. However, we're always looking for ways to improve Lucidchart, and your feedback is the best way for us to do that. Would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration. Thank you for your request and apologies for any inconvenience this may cause you.