Archive of our Own is the preferred website for a lot of fanfiction writers and fans. The interface is simple to use and navigate, the community is friendly, and it even allows users to download fanfiction PDFs for later reading. All good stuff.
Feedbooks has a nice digital library of original books fanfiction works from independent creators. Best of all, you can read them online or download them in EPUB, Kindle, or PDF formats and bring them with you wherever you go.
You may be wondering why Wattpad is on this list. Well, we know that it doesn't support PDF downloads but its app offers an offline mode that basically serves the same need. Give it a try. You might like it and even prefer it as a better solution.
Sweek is a new service for readers and writers alike. Similar to services like Wattpad, it provides an alternative to downloading fanfiction PDFs with its offline reading mode, which we think you'll find more convenient. It's still in Beta but it's already quite polished to use.