Confluence Cloud currently doesn't officially support PDF space exports over 2000 pages/1000 attachments. This isn't enforced in product but these exports are likely to fail.

Workaround

Export a smaller subset of pages using Custom Export. See https://support.atlassian.com/confluence-cloud/docs/export-content-to-word-pdf-html-and-xml/ 

To export pages to PDF, CSV, HTML, or XML

1. Select Space settings from the sidebar. 
2. Select Export space from the Manage space card.
3. Select PDF, then select Next.
4. Select Custom Export option, and select a smaller set of pages.
5. Select Export.