I know the majority of people don't let google crawl their WordPress attachment pages. In my opinion the attachment pages aren't duplicate content because they don't show the whole content but just a unique image or PDF link or other file. I don't understand why people hesitate to use them in a natural way.Although it will cause Google to index a lot of extra pages in your site, doesn't this  increase the visibility in search results?  What are the drawbacks of letting Google see these pages?
Is is good idea to let google list attachment pages?
seo;wordpress
It's not a good idea because the attachment pages usually contain no useful content or context for the user. While attachment pages may not generate a duplicate content penalty, they do make for a poor user experience and will probably raise your site's bounce rate metric.  Needless to say, you don't want that. It's pretty telling that on of the most popular SEO plugins for WordPress (Yoast) contains a setting that redirects attachment pages back to the parent page or post automatically.  That feature comes for a lot of users requesting it and there are good reasons to not want your attachment pages in the index. However, if you can think of a way to make the attachment pages relevant within the larger context of your site so that an inbound user doesn't immediately back away then you should let Google index.