Note that apps like WhatsApp and Facebook Messenger will notify the other people in the chat if you screenshot a message.

You want to quickly press them at the same time, then release them.

If you ignore the thumbnail for now, you can find your screenshot in your Photos app in Albums > Screenshots. [1] X Research source

Tap or ignore the thumbnail that appears at the bottom of your screen. If you want to edit or share the screenshot, you should tap the thumbnail before it disappears. You’ll be able to edit, crop, draw, erase, add arrows, and otherwise mark it up. Tap the Share icon and you can send the screenshot to your contacts. If you ignore the thumbnail for now, you can find your screenshot in your Photos app in Albums > Screenshots. [3] X Research source

Tapping this will change the screenshot to include the full page in Safari instead of what’s displayed on your screen.

When you’re finished, tap Done in the top left corner. Any full-page captures will be saved as PDFs in files instead of your Photo Gallery. [4] X Research source