Hello,
Do you want the survey to be truly anonymous? Or are you gathering information on who the participant is during the survey? If it is truly anonymous, there isn't a way to identify who they are when they revisit the survey at a later time. There are a few possibilities top of mind:

1. Distribute personal links via email (Email Survey Invitations). Ensure this setting (https://www.qualtrics.com/support/survey-platform/survey-module/survey-options/survey-experience/#AllowingRespondentsToSaveProgress) is turned on so they can revisit the survey in the future. Also, please check all 3 expiration settings to make sure they are set as you prefer (https://www.qualtrics.com/support/survey-platform/distributions-module/email-distribution/emails-overview/#DistributionAndSurveyExpiration).


2. If you want to use the anonymous link and QR code, I recommend using some sort of authentication that allows them to revisit. The value they enter to authenticate should be unique. And if it identifies who they are, it's no longer anonymous. Explore Authenticators (qualtrics.com). Make sure you review the section titled "RELOAD ANY PREVIOUSLY SAVED PROGRESS UPON AUTHENTICATION". Qualtrics recommends disabling Allow respondents to finish later.


3. Use the anonymous link and QR code, but do not allow revisiting. A survey that takes several sittings could have respondents quit before they get to the end. Try streamlining the questions as much as you can and only ask questions that give you results that you can take action on.


4. If you gather identifying information, you could redistribute a second survey only to those you'd like more information from.