Don't assume you have to convert all your existing documents into electronic files. This is expensive and difficult, particularly if you're taking the opportunity of installing a document management system to change your business processes.
Document management system suppliers will usually suggest that you choose a day from which you want all your new documents scanned. You then leave your current paper files as they are unless you need to work on them. In this event, you can scan them into the system.
If you adopt this approach, you won't have a backlog of scanning that you need to catch up with. You can also dispose of your current paper files at a later date when you feel you no longer need them. Alternatively, you may decide to convert the paper files as part of a special exercise when you've demonstrated that the document management system is worthwhile.
On the other hand, you may be in a business where one of the main advantages of having a document management system is that you can easily access every one of your current documents electronically. If so, you'll need the services of a company who can take all your documents and convert them. These companies have top-of-the-range scanning machines that can rapidly scan tens of thousands of pages.
Normally, such a company will remove your documents to their own premises, put the scanned results on disc or directly on your system, and destroy or return the documents as you wish. If the paper files contain sensitive material, however, you may want to keep them on your premises for scanning. Under this circumstance, the scanning company may have to install extra machines temporarily on your site.
Your document management supplier should be able to recommend a scanning company with proven experience. If you're interested in this option, ask about it when you contact document management system suppliers.