Our company is trying to send PDFs and documents to salesforce from a third party system. I need to know how can this be achieved. Do we use document service method , if not what this actually is ?
You'll need to use Data Loader or a 3rd Party Utility from the App Exchange to upload Attachments or Content into Salesforce as neither is supported by the import wizards at this time. It can be configured to extract data directly from your existing 3rd party database.
Instructions on how to do that are included in the document I've linked you to. Hopefully your 3rd party system admins will be able to help you with that especially since I'd expect PDFs to be a bit different than other data because of the encoding that's likely involved.
via email, you can use apex inbound mail handler. via intergration, you can use SOAP API or REST API.