I opened a pdf file from an unknown email. Should I be worried if my iPhone got hacked?

Mere clicking on a link does not affect much. Do not divulge any personal credentials. You are safe till then.

Clicking on links merely sends a response to the server hosting the link that a human is present. They can't do anything else except to keep sending such links again and again in the hope that you may give out some info.

1. Don't share sensitive information pertaining to your device and payment methods
2. Beware of Phishing