I don't recommend changing the resolution of the photos individually. That is a whole lot of effort for no real value if you are going to export to PDF.
When you create a PDF from a Word document you have options to change the resolution of the photographs. If your machine with v16.73 is not showing a SaveAs dialog where you can choose PDF then we need to work out how to do that. One of my machines has version 16.0.5395.1000 and the first screen gets me to choose a location but the the second screen asks for a filename and file type. On that second screen, once I choose PDF, I can Optimize for either Standard (better photo resolution) or Minimum size (probably 72dpi).
There are multiple ways to produce PDF output from Word and if you also use third party tools like Adobe Acrobat or Nuance PowerPDF they add functionality to MS Office so you have additional options to set image resolutions while creating PDFs. I would also expect that there are online tools which allow you to convert Word to PDF with options to set image resolution.
Finally, you might also be able to run a macro which converts a Word document to a PDF and specify the image resolution as part of it.
Code:
ActiveDocument.ExportAsFixedFormat OutputFileName:="C:\Users\username\Documents\Test Resolution.pdf", _
ExportFormat:=wdExportFormatPDF, OptimizeFor:=wdExportOptimizeForPrint, Range:=wdExportAllDocument, _
CreateBookmarks:=wdExportCreateHeadingBookmarks