How to open a large file with an .tar.xz extension?

7-zip has a nice, easy GUI that can let you extract .xz (and many other) archives.

XZ files are not associated by default, so after installing 7-zip, right-click your .xz file and select 7-Zip->Open archive Then you can double-click the .tar file in that to see the .pdf file. You can then open it directly or since it is so big, it would be better to extract the .pdf file somewhere first.

As for ExtractNow, yes it does have xz files in its list of supported archive formats. If you already have ExtractNow installed and are familiar with it, then you could certainly use that.

Quoting Wikipedia:

xz is a lossless data compression file format incorporating the LZMA2 compression algorithm. While xz can only support one file the convention is to bundle a file that is an archive itself, such as those created by the tar or cpio Unix programs.

You need to decompress the outer .xz archive, and then decompress the inner archive. You can use XZ Utils, as mentioned by @Journeyman, or 7-zip to extract the files. I recommend 7-zip, as it is more user-friendly.

Apparently you can use xzutils, which has a windows port. xzutils' readme also indicates 7zip will work