If you know the job / phase that a document is attached to, finding it may be as simple as opening the job, going to the attachments page and finding the document there. If you don't know the job, or even client, then finding it will be very difficult. Starting from Synergist 12.5 there is now a full attachments list available from the main navigation:
This opens a list that can be searched and filtered like other lists in Synergist. You can also select several attachments (by ticking the box next to them) and download all at once, instead of downloading them one at a time.