First, note that buyers worldwide can pay you in whatever your pricing currency is, and the payment processor (e.g. PayPal) or buyer's card issuer would automatically convert the payment amount to their native currency. Most sellers just set prices in their own native currency, or in the native currency for their primary target market -- e.g., if you primarily sell to buyers in the US, you might prefer to set prices in USD even if you're based outside the US.
Each product has its own setting for pricing currency, so if you'd prefer to set fixed prices in different currencies, you'd need to create duplicate products priced in each different currency. If you're using PDF Stamping, that would require you to upload a copy of your PDF to each duplicate product, but unstamped PDFs and other file types can use the "Bundle other products" setting to have the duplicates issue links for the file already uploaded to the original product.
Also, bear in mind the shopping cart can only handle one currency at a time (clicking Add to Cart for an item priced in a different currency would reset that buyer's cart and start over with that latest item), so you'd want to minimize any chances that a buyer could inadvertently click Add to Cart buttons for items priced in differing currencies -- e.g., your site navigation could ask buyers to select their currency and direct them to a dedicated sales page with purchase buttons using that currency, and no easy way to wander from that sales page to any other page with buttons using a different currency. Buy Now buttons would not have that problem, since they bypass the cart to take buyers directly to instant checkout for just one item at a time.
