What is the best way to update the purchase order in bulk without changing any data? It is to initiate the workflow.

Use CSV import and update at least one field value - it can be the existing value. In the import option expand the advanced options and select the run server script and workflow script check box, then run the update. Note your workflow must have trigger on update checked for this to work (which you can turn on temporarily). See the screenshot below

Thanks a lot

You can also initiate workflows using mass update. Also transition them in bulk using mass update

Thanks