For up to about 20,000 tickets, you can run this query from the Report Builder:
If you have added a lot of custom ticket fields, this may not work (Reports can only include up to about 55 fields). In that case, run a query which specifies the ticket fields individually. Replace SELECT tickets.* in the query above with:
SELECT tickets.id, tickets.subject
The full list of ticket fields is available on the Reports Manual.
You will then be able to export the result in PDF or CSV format.
If you have more tickets than the Report Builder can output, you will need to export your tickets using the DeskPRO API (requires programming skills).