For full document guidelines, please refer to this guide.
A. FLAT FILE
Access the Amazon Seller Central page by clicking the URL: https://sellercentral.amazon.com/
Once there, you will be asked to log in. Log in using your Amazon login details.
Once you are logged in, the landing page should look something like this:

NOTE: Update as of March 2024, putting documents in a product is now the same as you would update a file - via flat file.
On Seller Central's Home page, click the hamburger icon (top left) and go to Catalog > Add products via upload.

Click Download spreadsheet.

Select Download Product Spreadsheet.

Download the flat file depending on the category you want to edit - in this example, we will be using Soap Dish.


Inside the flat file, fill out the necessary details needed for a PartialUpdate, and look for this item in the flat file.

Put the URL of the certificate/manual that you want to attach
The file should be a PDF
The URL should be from a publicly-accessible link
Example: Save the PDF to the client's Google Drive folder, and get the shareable link ("anyone who has the link can view")
Upload the file in Seller Central. Once the file is uploaded and after the turnaround time, it should appear on the PDP.

B. BACKEND (MANAGE INVENTORY)
Access Inventory Management:
From the Inventory drop-down menu in Seller Central, select Manage All Inventory.

Edit SKU Details:
Click Edit next to the SKU for which you want to upload documents. This will open a new page in your browser.

Select Attribute View:
On the new page, choose All attributes from the left-hand menu (it may already be selected).

Suggest Changes (if applicable):
If the option is available, click Suggest changes to product information in the top-right corner of the page.
Go to Compliance Section:
Click on the Safety & compliance tab.

Complete Mandatory Fields:
Ensure all required fields are filled out.
Upload Compliance Documents:
Scroll to the Compliance media section and fill in the following details:
Compliance media language: Select the language code from the drop-down menu. Use the code for the store’s official language (e.g., fr_FR for France). For multilingual documents, select only the official language of the store.
Compliance media content type: Choose the document type from the drop-down menu. Refer to the "Permitted product documents" section of the help page for descriptions.
Compliance media source location: Enter a direct download link to the product document. Avoid using shareable links that open the PDF in a browser.

Add Additional Documents (if needed):
Click Add more to include any additional documents for the same SKU.
Save Changes:
Click Save and finish to save your updates.

VALIDATION:
Your product documents will be reviewed for compliance with Amazon's guidelines within seven business days. Documents that do not meet these guidelines will be rejected.
Once approved, it may take up to 24 business hours for the documents to be published on the product detail page of the corresponding SKU.