This is a database design question.

I want to build an invoice web application, an invoice can have many items, and each user can have an inventory list of product items that they can store and choose to add to an invoice item.

My questions are:
1. Should I store all product inventory for all users using my application under one single table? Or have a separate product inventory table created for each user?
2. Is this even possible?

1 table is easier, but what if this single table grows too big, will I have a problem? (primary key INT).