1. What does it mean?
팬아저 shortens 팬이 아니어도 저장. It is a compliment for an image or moment that appeals beyond an existing fandom: the speaker says it is worth keeping even without being a dedicated fan.
Also written or said as 팬 아니어도 저장, paenajeo
2. The nuance
It does not mean a fan who is an 아저씨, a middle-aged man. Saving something for yourself and having permission to republish it are different matters.
3. Examples
“팬은 아닌데 이 표정은 팬아저다.”
I'm not a fan, but that expression is too good not to save.
“사진 분위기가 좋아서 친구도 팬아저라고 했어.”
The photo's mood was so good that my friend wanted to save it despite not being a fan.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.