If PayPal isn't available in Bangladesh where you are based, the issue wouldn't be if someone from USA wanted to pay via PayPal, the issue would be you setting up PayPal on your store and being able to receive the funds. If it's not available where you live, then I don't believe it would work. Your best bet would be to use a payment gateway Shopify is partnered with in Bangladesh which you can see here. If you have one of the gateways listed set up with your store, someone from the USA would be able to pay and order from your store as it will available to anyone with a credit card. To learn more about PayPal and its availability in certain countries, it would be best to reach out to PayPal's support directly as this is something they would have control over, not Shopify.
I hope this helps and answers your question!
Nick | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog