Warning: I am not a tax professional. You should consult a tax attorney...yadda yadda yadda...
With that said, it totally depends. My online HSA account portal is electronically linked to my health plan, so the claim filed by the pharmacy (for price adjudication) will flow into my HSA account and can be easily linked up with the payment made through the debit card. In other words, all the documentation legwork is done for me. In other other words, you do need the documentation, but your HSA company and health plan might already be gathering this for you.