ManualsLib - Just enter the keywords in the search field and find what you are looking for!
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Internet Archive Manual Library - Internet Archive Manual Library is your go-to source for viewing the big collection of datasheets, instructions, and guides written for a wide range of products.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Manuzoid - Repository that includes user manuals for various products
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.