There are several options that depend on what client experience you want to give your clients
Your choices:
Option 1: I don't want clients to have to create a login
Go to Documents, createa a Form with 'file upload' widget. Then share it via email or a link. The client will receive an email with a link to the form, the form will open in a browser. The client will then be able to drag/drop the document or click 'choose file' to select it. If you need the client to upload more than 1 file, embed the 'file upload' widget in a 'section' widget and specify the maximum number of times your client will be able to 'clone it'
Pros: useful for a fixed number of documents
Cons: requires a single sitting, once a form is submitted it cannot be undone or changed. All submissions are stored with the form's template as separate items and may be difficult to identify.
Option 2: I want clients to be able to save their work and come back to it or I want them to upload files at multiple occasions
You need a client portal. Go to Portals, click on '+' and cread a Client Portal Group. Then click 'add new' and create a portal (you can invite the client at this step). For more options on how to create and configure portals, go here.
Note that this option requires your clients to create a login to the portal. However, they will not need to login every time you ask them to upload documents. To ask your clients to upload documents, go to their portal, click on 'messages' and create a 'document upload request' message type.
The client will receive an email with a secure upload link which does not require them to login to the portal first.
If they select 'upload documents' button, they will be taken to a browser window through which they can drag/drop documents that will be stored in their portal's document area.
If they select 'open portal [name]' they will need to login first.