The library lives alongside your templates and images in Design & Templates. Each document has a name and a description, so "Onboarding Guide" is easy to find even when the file itself is called og-v7-final-FINAL.pdf — and the whole library is searchable by name, description, or file name.
When a document changes — new fees, new wording, new branding — replace it in the library and every workflow step that references it sends the new version from then on. No hunting through workflow templates for stale attachments.
Practice documents are sent exactly as uploaded. That's the point: they're your fixed, polished files. For documents that should be generated per client with their own data merged in, use document templates instead.