Every donor gets a receipt showing how much was donated and to which fundraiser. There are two ways to get it.
Straight after donating
The donation flow has four steps: Donation, Details, Payment and Tax Receipt. On that last step there is a Download Receipt button.
From your confirmation email
If you entered an email address, the confirmation email contains a Download your receipt button. For a recurring donation the wording is Download my receipt, and you get one for each payment.
That link keeps working. It is worth keeping the email, or bookmarking the link, because it is the simplest way back to your receipt later.
If the email arrived without a receipt button, it will say instead to keep the email itself as confirmation of your donation.
There is no donor account page for receipts
WhyDonate does not have a "my donations" page where past receipts are listed. The email link is the route back to an old receipt.
If you cannot find the email, contact support with the email address you donated with and roughly when you donated.
For organizers
If you receive donations, you can download a receipt for any individual donation from your balance overview, by opening the transaction and choosing Download Receipt.