Yes, just per file creation.
If files need to be sent by email,
then + for each email.
You pay only for use.
No monthly subscriptions.
No. You pay only for use - for creating files, emailing them, and storing them.
Yes. After registering you will receive 100 welcome credits, which you can spend on 100 file creation. There is no time limit for testing.
No. You can top up your balance for the amount you want and use these funds for any length of time.