Let's make sure that we've ticked the option to include PDF on the invoice, vassili-gorshkov.
Let me guide you with these steps:
- From the Gear icon, select Account and settings.
- Go to the Sales tab.
- Click the Online delivery section.
- Tick the PDF Attached box.
- Click Save and then Done.

If you want to let your customers pay the invoice online, you can send an online invoice: Take and Process Payments Online.
I'm just a post-away if there's anything that I can help. Keep safe!