"I found many companies with PLC in their name"

In the UK, "PLC" in a company name means "Public Limited Company" - ie, one traded publicly on the stock market.
This was introduced a few years ago to distinguish from "Ltd" (short for Limited) which now indicates a non-publicly-traded Limited company.

This all goes to show how important it is for people to define any abbreviations they use - not everyone can be familiar with all abbreviation, and many abbreviations have different meanings in different contexts.
eg, see http://www.8052.com/forum/read.phtml?id=24190

Andy Neil,
Antronics Ltd.