If you've already uploaded a file to a client's CRM Document storage, you can link it to one of their existing accounts. Instructions are here.
If this is not a file you've already uploaded, however, you can upload it from the Account Details page of the account to which you want it linked. Note: the file will be accessible from within their CRM Document Storage area as well.
To upload in this manner, go to the contact record for whom you wish to upload the file and go to their Accounts area.
Then, either double-click the account in question or right-click it and select View Account Details.
On the resulting page, scroll down to the Account Documents widget:
Click Add. You'll then see a dialog for browsing for and uploading your file:
From here, click Add Files. You can then browse for the file you want to upload (Note: your browse dialog may look different than the example below):
Click the file you want to upload and then click Open:
Click Start Upload and the file will be uploaded and linked to the account once complete. Your Upload Document dialog will then appear as the below:
Just click the "X" to exit, or add more files to link to the account if desired. If we were now through with linking files to this account, the Account Documents widget might look like the below:
You have the below right-click options for documents in this area: