LTV (Long Term Validation) have a long expiration date, but it may be NOT forever, so I think we need to take appropriate action after the life-time is exceeded.
The specification of the downloaded PDF seems to be PAdES-LTV.
In PAdES-LTV, I understand that the expiration date of the entire PDF is the expiration date of the last added document timestamp.
When the certificate of document time stamp attached to the downloaded PDF expire?
If it's long enough, I don't have to do anything, so I asked how long it is.
I looked at page 9 of the PAdES-LTV specification (below link) and understood and asked the question as above.
https://www.etsi.org/deliver/etsi_ts/102700_102799/10277804/01.01.01_60/ts_10277804v010101p.pdf