=>, <=, and <- are rendered as invisible in the PDF. These “replacement substitutions” aka “textual symbol replacements” ( http://asciidoctor.org/docs/asciidoc-writers-guide/#replacements ) render as invisible characters in the PDF, though they are visible in the HTML format, when substitution is in effect.
This is only a problem outside of code blocks (e.g. --> close comment syntax in XML is never improperly substituted).
In the screenshot below, a table from the CharFilterFactories|solr.MappingCharFilterFactory section is shown in the HTML format on the left, and in PDF format to the right; in the PDF format the => is invisible.