Soon it will be time for Japanese individuals (freelancers or sole-proprietors) to file their tax returns.
One of the important documents to submit for filing is the Payment Certificate. Generally, Japanese companies send these to their outsourcing workers around January.
However, it’s possible that you may not receive them, as companies are not required to send them. They are only for the tax office. However, you can request them or use your other documents such as bank statements.
Just remember to check that the amounts match, that the tax has been calculated correctly and that the documents are accurate.