Follow the below steps to add a button to any object:
1. Go to the Setup->Customize
2. Click on Accounts (OR any object where the button should be added)->Buttons, Links, and Actions
3. Now Click on "New Button or Link".
4. Provide the following information on the above screen:
- Label: Download Attachment (you can change it if required)
- Name: Download_Attachment (you can change it if required)
- Description : Optional
- Display Type: Detail Page Link or Detail Page Button
- Behavior: Display in existing window without sidebar or header
- Content Source: URL
- Formula Editor : /apex/DownloadFiles__MassFileDownload?parentid={!Account.Id} (Copy that formula from the About page, Select-Object, and then copy the formula Text as shown in the below image)
- Then click on the "Save" button.
5. Finally, add the button to the required page layouts of the account (or object for which this button was created) and it's ready for use.
- Setup --> Account -->Page Layouts
- Select Layout where you want to add the "Download Attachment" Button and click on "Edit".
- Buttons --> Drag "Download Attachment" in Custtom Button for Classic UI and on "Salesforce Mobile and Lightning Experience Actions" for Lightning UI, so you can use it in Lightning.
6. Now hit the "Save" Button. 