Having Automator send downloaded files to DEVONThink
This Folder Action will automatically import any files that are downloaded into your Downloads folder into your DT database.

Create a Folder Action in Automator and after pointing it to your 'Downloads' folder add the following Actions to it: 

Get Folder Contents
 Set Current Group (and then select the desired folder within your DT database) 
Add Items to Current Group

If you wish to take a more manual approach, you could create a 'service' in Automator with the following actions: 

Get Specified Finder Items (and then 'Add…' your Downloads folder) 
Get Folder Contents 
Set Current Group (and then select the desired folder within your DT database) 
Add Items to Current Group (and then select 'import')

You can then right-click the Downloads folder and run this from the Services sub-menu. You could also add a keyboard shortcut in System Preferences > Keyboard > Shortcuts > Services.

NB For the ‘get specified’ action, you may need to check the ‘ignore this action’s input’ or get duplicates. Set as follows: 'Service receives selected ‘folders’ in ‘Finder’’.

As an alternative, you could substitute ‘get selected finder items’ for the ‘get specified…’ above. Then select all the items in the Downloads folder when you run the service. This would allow you to ignore some files if you don’t actually want to import them to DT. 
You have a few options depending upon how you would prefer it to work. For example, you could add a ‘Move Finder Items to Trash’ action which would presumably trash the files after importing them into DT.