DMCA is often used to take down service manuals. Try to find the service/repair manual for a Pentax K20D, you'll end up in the darkest corners of the internet.
But even without the DMCA, having unauthorized copies of copyrighted documents is still illegal, right?
The DMCA creates a liability shield for sites that accept uploads and creates a procedure for takedowns. But without the DMCA, the company's lawyers could still send a nasty letter to the web hosting site, and they'd be even more incentivized to prevent unreviewed uploads from users in the first place because they wouldn't have the liability shield.
Also, the takedown / liability shield parts of the DMCA are also only relevant when the people running the site are not the people who uploaded the content. If you want to host your own website and claim that you're republishing the manuals because of fair use, that part of the DMCA doesn't affect you at all.
(The DMCA does meaningfully interact with right-to-repair in its anti-circumvention provisions, though.)
I don't think the original poster is talking about the DMCA copyright takedown notice/counter-notice procedure. They're probably talking about the DMCA's anti-circumvention bits, which make distributing tools to circumvent copyright protections (even on hardware you own) illegal.
On 30 November 1999, the U.S. District Court in Seattle, Washington, dismissed Mackie claims that Behringer had infringed on Mackie copyrights with its MX 8000 mixer, noting that circuit schematics are not covered by copyright laws.
Data has never been copyrightable. Copyright has only ever protected creative expression. Draw your own diagrams and write your own explanations and you're fine. (See iFixit, for instance.)
It's just like how you can't post a copy of The Matrix online but you can definitely write a plot summary without infringing anyone's copyright. Whether it makes sense that manuals (and firmware, for that matter) are considered as worthy of copyright protection as The Matrix is a separate question, but the law sees them the same way.
The DMCA creates a liability shield for sites that accept uploads and creates a procedure for takedowns. But without the DMCA, the company's lawyers could still send a nasty letter to the web hosting site, and they'd be even more incentivized to prevent unreviewed uploads from users in the first place because they wouldn't have the liability shield.
Also, the takedown / liability shield parts of the DMCA are also only relevant when the people running the site are not the people who uploaded the content. If you want to host your own website and claim that you're republishing the manuals because of fair use, that part of the DMCA doesn't affect you at all.
(The DMCA does meaningfully interact with right-to-repair in its anti-circumvention provisions, though.)