To upload a document file against a deliverable or a task, first you need to find the 'Deliverable Details' page and then upload file in correct folder.
On WorkPack dashboard, you can find the task / deliverable against which you want to upload a document. Double click on that task to open the 'deliverable details' page for it.
The top of the 'Deliverable Details' page displays basic details of that task / deliverable such as - title, deliverable number, project name & number, deliverable owner and scheduled dates for approval, submission etc.
In the lower section, you can see different folders where you can upload your document files. Select the proper folder by clicking on the tab. Then browse files from your machine and upload them to the selected folder.