One place for the policies and procedures your forms refer to.
Documents → Add. Give it a name, a version and a review date, and upload the file (PDF, Office documents, images and videos are previewed in the browser). Use Settings → Document types to group them, and Third-party documents for things you did not write yourself, such as supplier certificates.
Add a Document question to a form and pick a document: it is shown for reference while the form is filled, including on public links. Handy for a procedure the checklist is verifying.
Each document has a review date and a review history. Record a review when you have checked it is still current.
The free plan includes 100 MB of document storage; paid plans include full storage. You can see the usage under Plan & billing. Photos attached to submissions do not count toward the document limit.