articles with ordinal numbers

I thought that ordinal numbers can have only a definite article. Then I read one post yesterday from a native speaker where he used indefinite article...

"A second text file was used to pry it open"

Why he used indefinite article?

I've also found out that ordinal numbers don't need to have an article at all.

When is then used definite, when indefinite and when zero article with oridnal numbers? 

Damir

I thought that ordinal numbers can have only a definite article.

You are learning English as *a* second language, aren't you?

A second look.  A second opinion.  A second chance.