By default, OpenCRM does not automatically save and attach PDF's that are generated. However you can enable this by going to Settings > Additional Settings > PDF Settings where you should find an option titled 'Attach Exported PDFs to the parent module as Documents'.
By ticking this option every PDF that is generated will be saved and linked to the record it was generated from. You can view these PDFs in the Documents sub-tab on the relevant record.
This feature applies to Sales Orders, Quotes, Invoice, Helpdesk, Projects and Companies.
Please note that Despatch Notes are an exception - when a Despatch Note PDF is generated it will always attach the PDF to the sales order it was generated from.