John564479 Deleting the secondary user profile and then rebooting the phone is the only way to be sure all traces of the documents are gone. That is because the encryption keys for the secondary user profile is destroyed by that operation.
Just deleting the documents or the whole folder just marks that storage space as available for reuse. The data is not actually deleted, and can be recovered using advanced forensics by someone who knows your passphrase. There may also be other metadata such as thumbnail pictures that aren't deleted just because the file itself was. The only way to be sure is to delete the whole user profile.
But I doubt security at an airport would be performing any advanced forensics, it would require disassembling the phone and would take a lot of time. So if you just worry about someone using the user interface in a normal way, deleting the folder with the files and then rebooting the phone is probably enough, unless you have opened the documents in an app that keeps a list of recently opened files or something like that. Still, deleting the user profile entirely is the safest option.