When creating or editing Activities, there is the option to upload a Document as an Attachment.
You can use the folder icon to select a document already uploaded to OpenCRM, or the Choose File option to upload a new Document.
There are two fields - one for existing documents, and one for new files.
If you wish to add multiple documents use the + symbol, and if you need to remove an attachment (prior to upload) use the - symbol.
On Save those documents will be visible in both the Attachment field itself and also in the Documents sub-tab:
If you need to remove a Document that has been saved against an Activity you can use the del link tool which is on the right side of the Documents sub-tab.
If you have linked multiple Documents linked to an Activity, you can download them either individually by clicking on the file name, or clicking on the arrow offers you the choice of downloading the files individually or a a zip file.
Note that you can also link Attachments to follow-up (FUP) activities.