Anyone can submit documents there, including manuals.
- you scan or photograph the pages of a printed manual, assemble them into a PDF and upload them.
- The Internet Archive performs optical character recognition (OCR) on the images
- both versions of your manual, OCR text and original PDF appear on the site
- your manual is searchable, and will almost certainly appear in Google searches (in time) as well as searches within the Internet Archive (immediately)
For operational reasons very few people are allowed to upload directly to the Manuals Collection, but that's not a problem, just upload your manual as "community text" and it will appear in searches of the site.
1) If you do not already have an account with the internet archive, go to archive.org and create one 
2) It's really easy to upload a manual - click upload and follow the prompts. When it asks you where you wish to submit your manual (i.e. which collection) you will want to choose "Community Texts" option from the drop-down.
3) Whenever you log in to your account on archive.org you will see a collection of the manuals you have submitted, this is potentially a really good way of saving shelf or disk space wherever you keep your manuals, you will also be doing the rest of the world a favour.
3) There is a process to move documents into the Manuals Collection, but you might not want to bother with this since your manual will appear in web searches anyway How to move an item to another collection (ext. link)