Web: You can automate your Dropbox with Wappwolf, including converting PDF files to TXT, but if you need your PDFs as Doc files to open in MS Word or other word processors, Convertii is the way to go.
You'll need to sign into Convertii to use this feature, and grant it permission to your Dropbox. It creates a new folder in your "Apps" folder. Upload or move your PDF file there and Convertii will automatically pick it up, convert it to Doc and place the new file there. It's automated, but on the first few tries, it took some time executing the conversion for me — 10 minutes, roughly. However, after the first day, it started doing it at the same speed as Wappwolf takes for other conversions.
It's simple and fast, but like most PDF to Doc conversions, you can expect some of the formatting to be disturbed when it comes to tables and charts.