The contents readed by read document operator from PDF file are disorder

Lei

I use read document operator to read a PDF file. The content order is different from the text order of the PDF file itself.

<?xml version="1.0" encoding="UTF-8"?><process version="9.10.000">

  <context>

    <input/>

    <output/>

    <macros/>

  </context>

  <operator activated="true" class="process" compatibility="9.10.000" expanded="true" name="Process">

    <parameter key="logverbosity" value="init"/>

    <parameter key="random_seed" value="2001"/>

    <parameter key="send_mail" value="never"/>

    <parameter key="notification_email" value=""/>

    <parameter key="process_duration_for_mail" value="30"/>

    <parameter key="encoding" value="SYSTEM"/>

    <process expanded="true">

      <operator activated="true" class="text:read_document" compatibility="9.3.001" expanded="true" height="68" name="Read Document" width="90" x="112" y="85">

        <parameter key="file" value="D:/water electrolysis/PDF electrolyte/0019178499_#14. 1-s2.0-S0925838817304759-main1.pdf"/>

        <parameter key="extract_text_only" value="true"/>

        <parameter key="use_file_extension_as_type" value="true"/>

        <parameter key="content_type" value="txt"/>

        <parameter key="encoding" value="SYSTEM"/>

      </operator>

      <connect from_op="Read Document" from_port="output" to_port="result 1"/>

      <portSpacing port="source_input 1" spacing="0"/>

      <portSpacing port="sink_result 1" spacing="0"/>

      <portSpacing port="sink_result 2" spacing="0"/>

      <portSpacing port="sink_result 3" spacing="0"/>

    </process>

  </operator>

</process>

Extracted by read document operator: 


Original PDF file:



 The order of highlight words in above two images is totally different. 
Could someone help me to fix this problem? Thank you very much.