The passport is a pretty nifty symbol. It's the key to Harriet's successful disappearance. Here's the basic scenario. Anita loans Harriet her passport so she can travel. Harriet marries Spencer Cochran under Anita's name. So, legally, Anita is married to Spencer, though she uses the name Vanger. Harriet gets a new passport, in the name of Anita Cochran.
We don't know how this whole thing works for taxes and such, but essentially Harriet and Anita share the same identity. Since Harriet/Anita's marriage to Spencer is registered in Sweden, if anybody had ever looked up Anita Vanger in the Swedish national registry, they would have found Harriet's address. As such, the passport becomes a symbol of a few things:
- Doubling and Tripling: Horror and mystery stories love doubles, and twins and doppelgängers, because such stories explore mistaken identity and the multifaceted nature/identity of human beings. The passport signals us to those aspects of the story.
When Blomkvist learns that pictures he thinks are of Cecilia, but are really of Anita (Cecilia's younger sister) Harriet's doubling with Anita is foreshadowed. Cecilia and Anita are almost identical in looks.
And Harriet is almost identical to them, except she has brown hair and they have blond. Harriet easily hides her roots, just like Salander hides her Pippi Longstocking locks (as discussed in her "Character Analysis") with dye. Watch out for more Cecilia-Harriet-Anita doubling and tripling in the next books in the series.
- Friendship: Anita literally saves Harriet's life by giving her the passport, and letting her share her identity. If letting somebody share your identity, and keeping their secrets isn't friendship, we don't know what is. As a symbol of their shared identity, the passport is a symbol of their friendship.
- Freedom/Imprisonment: Most obviously, the passport is a symbol of freedom and imprisonment. It gives Harriet the freedom to travel, to hide, to live a Martin-free life. But, because it's an identity shared with another person, and one she had to take on to hide her past and the killing of her father, it's a symbol that she's imprisoned by secrets. At the end of the novel, she seems to be picking back up her old identity – though we aren't sure how she'll explain all this to her kids.