I have a site where I allow clients to either view material online, or download it to their computer. The ability to download is a must, as some clients have limited access to internet. These files are either PDFs or mp3s. I am wondering if there is a way for me to stop them from redistributing the files (copying, sending them to other people, etc)?
The closest to a DRM solution for PDF files are likely to be Adobe's Digital Editions: https://en.wikipedia.org/wiki/Adobe_Digital_Editions It is mainly centered around ebook usage, and it's support might not be ideal for your usecase. But in terms of actual DRM for PDFs, this is likely to be most comprehensive solution. Depending on what your content is, it is likely it's not worth it. Especially since it doesn't really support browsers in the way that most use them today.
For audio, you can really go with any video DRM. Some of them might not support pure audio files, but having a small black only video wouldn't be a major problem to overcome. If you want wide browser support, you are likely to require at least Microsoft PlayReady and Google Widevine with an optional Apple FairPlay. You can get all of that from a single vendor such as https://drmtoday.com/