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
- Select Space settings from the sidebar.
- Select Export space from the Manage space card.
- Select PDF, then select Next.
- Select Custom Export option, and select a smaller set of pages.
- Select Export.
Suggestion
