Proper Way to Read a Data File in Javascript/Node.js?

I have a flat data file in the form of xml, but there isn't a real Windows viewer for the file, currently. I decided to create a simple application with Node-WebKit, just for basic viewing - the data file won't need to be written to by the application.

My problem is, I don't know the proper way to read a large file. The data file is a backup of phone SMS's and MMS's, and the MMS entries contain Base64 image strings where applicable - so, the file gets pretty big, with large amounts of images (generallly, around 250mb). I didn't create/format the original data in the file, so I can't modify it's structure.

So, the question is - assuming I already have a way to parse the XML into JavaScript objects, should I, a) Parse the entire file when the application is first run, storing an array of objects in memory for the duration of the applications lifetime, or b) Read through the entire file each time I want to extract a conversation (all of the messages with a specific outgoing or incoming number), and only store that data in memory, or c) Employ some alternate, more efficient, solution that I don't know about yet.