Word document changes after saving as pdf

You did not say what method you use to create the PDF other than saving as, but if the Word document is being changed, you likely have some type of plugin being used, like Adobe Acrobat.

If the plugin is having trouble with this one particular document, 'printing to PDF' instead of 'saving as PDF' should resolve your issue, but I have seen where an older, no longer supported version of Adobe Acrobat will output the same thing when 'printing to PDF' that the plugin did, so you might have to use Microsoft Print to PDF or other method of printing to PDF that is different from your plugin.

If this document is only the first exposure to the problem and you find that the output is getting changed regularly, the office plugin from the PDF software you use is the most likely culprit and should be uninstalled/replaced.