The UK has updated their content design guidelines in major way, and I suspect similar guidelines will find their way to the US before too long.
Specifically, they now say:
If you publish a PDF or other non-HTML document without an accessible version, you may be breaking the law.
The reason for this is pretty simple — they can be very difficult for some users to manage, as they tend not to be mobile responsive, may not work with screen readers, and difficult to increase font size as an end user.
I first mentioned this more than 15 years ago, that PDFs are only ideal for documents that are intended to be printed. Using PDFs as a quick way to add content to your site was a bad idea back then, and it’s a worse idea now.
Changing your website to accommodate that could be a very big deal if you have a lot of documents, so you’d be wise to start attacking this problem before any legal legs get behind it, as they likely will in the coming years.
You don’t have to trash them all
This isn’t to say that you need to get rid of all PDFs from your site. They can serve a purpose, particularly for items that need to be printed in a precise fashion, such as legal forms. Just be sure that any PDF has an open format alternative (usually just HTML, like this text) so that all visitors on your site can see what you have to say.
If you need further help, check out this post for more quick accessibility tips.