Bitrix Intranet Portal comes fully configured to support search in MS Office documents. If you want to enable your users to search documents in some other format, you have to provide an appropriate document preprocessor that would convert these documents to plain text. A preprocessor can be a utility that takes a file on input and outputs a plain UTF-8 text stream.

Obviously, you have to make Bitrix Intranet Portal aware of the new file format. Open Settings > System Settings > Module Settings > Intranet. Click the Search tab. Find an empty row and type the format extension in the Extension column, for example, djvu.

In the External Command column, type the command line in the following format: <executable> <file_name_ macro>. The converter must output the text in UTF-8 format.

For example, the following command can be used for djvu files and djvu2text converter:
djvu2text #SHORT_FILE_NAME# -d utf-8

In the Program Folder column, specify the executable file path. Save changes.

Now, add the djvu extension to the indexable files: open Settings > System Settings > Module Settings > Search and add *.djvu to Inclusion Mask.