Unfortunately, as much as we want to know there is a single benchmark out there, it would be impossible to provide a number that would be meaningful for your website. The meaning of a conversion rate number heavily depends on the preferences and motivations of your website visitors–which you may be able to group into segments, or measure in terms of “lead quality” or something similar. Therefore, only data from websites that get the same type of visitor would provide you with a potentially meaningful average–not all websites that offer white paper downloads.

Also, different websites have different objectives. While your question defines the objective pretty narrowly, even “whitepaper download” could mean different things and be prominent to a different extent on different sites–all leading to a conversion rate not being comparable apples-to-apples.

So, that’s a long non-answer to your question. However, the good news is that if you have determined that whitepaper downloads are important to you, it doesn’t matter where you are in comparison with other websites. In my experience doing consulting and research work, we start with whatever the conversion rate is, and improve it from there–and it doesn’t really matter if it’s “good” or not. Only testing can show you if it can be improved, and that’s probably your real objective.

So while this doesn’t directly answer your question, I hope I helped set aside the underlying concern.