You should split a large file or else let the
texinfo-format-buffer or makeinfo-buffer command do it
for you automatically. (Generally you will let one of the formatting
commands do this job for you. See section Creating an Info File.)
The split-off files are called the indirect subfiles.
Info files are split to save memory. With smaller files, Emacs does not have make such a large buffer to hold the information.
If an Info file has more than 30 nodes, you should also make a tag
table for it. See section Running Info-validate, for information
about creating a tag table. (Again, tag tables are usually created
automatically by the formatting command; you only need to create a tag
table yourself if you are doing the job manually. Most likely, you
will do this for a large, unsplit file on which you have run
Info-validate.)
Visit the Info file you wish to tagify and split and type the two commands:
M-x Info-tagify M-x Info-split
(Note that the `I' in `Info' is upper case.)
When you use the Info-split command, the buffer is modified into a
(small) Info file which lists the indirect subfiles. This file should be
saved in place of the original visited file. The indirect subfiles are
written in the same directory the original file is in, with names generated
by appending `-' and a number to the original file name.
The primary file still functions as an Info file, but it contains just the tag table and a directory of subfiles.