If you connect your Teamwork Projects site to your Dropbox account, you can set it up on individual projects so that you can link to your files from Dropbox directly in the files area of the project, or attach them to tasks or messages like you would with any files you've uploaded directly.
First, you'll need to authorise Teamwork Projects to connect to your Dropbox account. Go to the site settings menu in the top right of the screen and choose the 'Integrations' section:
In the Dropbox section, click the button to connect your account:
Once you've completed authorisation, you'll see another button to choose a top level folder. This allows you to control which files in your Dropbox account are accessible from Teamwork Projects, when you connect each project to your Dropbox, you'll be able to choose this folder for the specific project or set the project folder as any of the ones within this root folder - so it's a good idea to set up a folder for this purpose.
Click the 'Choose Folder' button and you'll see a list of folders in your Dropbox. Choose one, and click the 'Select this folder' button.
Now, you can enable the Dropbox integration on individual projects so you can link your files.