PDF manuals
PDF manuals, guides, data sheets are a valuable resource which is poorly archived and tends to disappear easily because companies die or simply don't care about their out of commerce products.
The main (accessible) resource in Internet Archive is https://archive.org/details/manuals but a series of manual websites have been preserved within web crawls.
Other archival projects
The web is infested with various domains which crawl manuals and compete for the best positioning in search results for very possible product code. Most of them try to monetize the clicks by obfuscating the PDF or other scam-like techniques.
Manuals can often escape the normal wayback machine crawling because the respective domains require JavaScript searches or even login (for instance on Electrolux websites).