I need to know the way to save a 450mb .txt file into my Kindle with 170mb of free space without deleting other files.

Defrag your disc. (Assuming your Kindle has such a thing, which I know nothing about.) Just a shot in the dark here…

Even if you were to ZIP the file (to compress it), when you want to open the file (to read or use it) you won’t have the space. So it’s going to fail to be available on the Kindle no matter what.

Unless you’re trying to compress it and use the Kindle as a sneaky way to do file storage, but that’s a whole different matter. Buy a thumb drive for that.

What are you really trying to do?

Read a bunch of ebooks in the kindle, but it has low storage space.

Limited to 50mb download to Kindle. . . trying to open a zip file might brick your Kindle !