getting pdf files to apple wallet and/or keeping critical data on my iPhone

I was in California recently (Paso Robles) and I was asked to show proof of vaccination. I went to iBooks on my iPhone where I had stored my scanned copy of my vaccine card, but it had been off-loaded to the cloud. And…there was no cell service or Wi-Fi. So I could not produce the documentation. I have scanned many things to iBooks, like theater tickets. I was not happy to realize that these files were no longer on my phone and that I need an internet connection to get them back.

I have a 13 pro with 256 Gb. I am using ⅓ of my storage. Yet my data is being off loaded. I was told that

I should have put my vaccine card in my Wallet since the files there are never off loaded, but I can't see any obvious way to do it.

Thanks for any advice.