doc-base files must be valid UTF-8, an encoding of the Unicode character set.
There are many ways to convert a doc-base file from an obsolete encoding like ISO-8859-1. You may, for example, use "iconv" like:
$ iconv -f ISO-8859-1 -t UTF-8 doc-base > doc-base.new $ mv doc-base.new doc-base
Refer to Debian doc-base Manual section 2.3.2 (Syntax of the control file) for details.