The default print version of this book is provided in the letter paper size. If you would like to have the print version of this book on paper and you are living in a country which uses A4, then you can rebuild the book. The great thing about the GNU build system is that the book source code which is in Texinfo is also distributed with the program source code, enabling you to do such customization (hacking).
In order to change the paper size, you will need to have GNU Texinfo installed. Open doc/gnuastro.texi with any text editor. This is the source file that created this book. In the first few lines you will see this line:
In Texinfo, a line is commented with
@c. Therefore, uncomment this
line by deleting the first two characters such that it changes to:
Save the file and close it. You can now run
$ make pdf
and the new PDF book will be available in
SRCdir/doc/gnuastro.pdf. By changing the
$ make pdf to
dvi you can have the
book in those formats. Note that you can do this for any book that
is in Texinfo format, they might not have
@afourpaper line, so
you can add it close to the top of the Texinfo source file.