"Have you seen it?" would be more appropriate. Here's why: "Have you seen it?" is in the present perfect tense, which is used to ask about an action that occurred at an unspecified time in the past but is relevant to the present. Since you're inquiring whether your friend has watched the reel (but you're not focused on when exactly they saw it), this form works best. "Did you see that?", in contrast, is in the simple past tense, which is typically used when referring to a specific time in the past. If you were asking about something that happened in a definite moment (like a specific scene or moment you both experienced at the same time), then this would be more appropriate.

If you are sending something and asking if the person has seen it yet/already, you would more typically use 'this.' Have you seen this? Did you see this? You could add 'yet' to the end of either questions, also.

Did you see it? Have you seen it? both are acceptable.