Documents sent through Amazon's Personal Document Delivery Service are automatically archived in your Kindle Library (you can control this from the Content Library page in your Amazon account). So even if you originally sent the file to a different Kindle device or app, you can re-send to another device all within your Amazon account.
Go to your Content Library page in your Amazon account, then under Your Content filter to see Docs and if the book appears there, click on Actions and then click on "Deliver" and select the Kindle device/app you're using. Your device will sync wirelessly and the book will show up on your Kindle device or app.