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Suppose that in your DITA content you have two block elements, such as two paragraphs:
<p>First para</p>
<p>Second para</p>

and you want to force a page break between them in the PDF output.

Here is how you can implement a DITA Open Toolkit plugin that would achieve this:
  1. Define your custom processing instruction that marks the place where a page break should be inserted in the PDF, for example:
    <p>First para</p>
    <p>Second para</p>
  2. Locate the DITA Open Toolkit distribution and in the plugins directory create a new plugin folder (for example, DITA-OT-DIR/plugins/pdf-page-break).
  3. In this new folder, create a new plugin.xml file with the following content:
    <plugin id="com.yourpackage.pagebreak">
      <feature extension="package.support.name" value="Force Page Break Plugin"/>
      <feature extension="package.support.email" value="support@youremail.com"/>
      <feature extension="package.version" value="1.0.0"/>
      <feature extension="dita.xsl.xslfo" value="pageBreak.xsl" type="file"/>

    The most important feature in the plugin is that it will add a new XSLT stylesheet to the XSL processing that produces the PDF content.

  4. In the same folder, create an XSLT stylesheet named pageBreak.xsl with the following content:
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
                          xmlns:fo="http://www.w3.org/1999/XSL/Format" version="1.0">
      <xsl:template match="processing-instruction('pagebreak')">
        <fo:block break-after="page"/>

The source code for the plugin can be found on GitHub here: https://github.com/dita-community/org.dita-community.pdf-page-break.