On the Travel Forum, I've read very helpful posts regarding difficulty in uploading CDC cards to Delta's FlyReady's app prior to international flights. I've just successfully done this so wanted to post how I did it. For those who are tech savvy, please forgive the simplicity of the steps below. I'm sure there are shortcuts.
Step 1: Scanned CDC card to Notes app on I-Phone
Open I-Phone's Notes app
Start a new note
Tap camera icon at bottom to take a photo
Tap on scan documents
Hold phone over CDC card
End result: have a PDF file of your CDC card
Optional: tap on 3 dots in corner then select Pin. Your scanned CDC card is now your 1st note so is easily accessible in the future
Step 2: Emailed PDF file of CDC card to myself
Open I-Phone's Note app
Select the note containing CDC card
Tap circle with 3 dots in upper right corner
Tap Send a copy
Tap Mail icon, then send it to your email address
Step 3: Saved PDF file of CDC card to my laptop (one reason I wanted it on my laptop was b/c I need it for a British Airways flight as well)
Opened email containing PDF file
Saved PDF file to my laptop
Step 4: Uploaded PDF file of CDC card to Delta FlyReady
Step 5: Within 5 minutes, received email from Delta saying "We've verified your vaccine record."
I looked at my Delta reservation for a different fight later in the summer, and it appears FlyReady is "good to go" there as well, so I don't have to repeat the process for future flights.