There are a number of issues with her stance, starting with the First Amendment:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievancesIt doesn't say anything in there about getting to choose what counts as protected speech. Otherwise politicians would be banning all sorts of dangerous speech, such as pretty much everything that Dr. Martin Luther King said. That isn't a can of worms that anyone in their right mind wants to open.
The next problem is the technical issues with removing it from the internet. The Anarchist Cookbook (and it's cousin the Anarchist's Cookbook) are both available from hundreds of sources on the internet, ranging from Amazon to The Pirate Bay and its contemporaries. There are a lot of downloaded copies of the book, and Senator Feinstein doesn't seem to realize just how hard it is to prevent people from sharing their copies with other people. Heck, if two parties are using steganography, the Feds might not even realize that someone's sharing the file.