Losing Image DPI/Resolution on hundreds of images - Can I batch update image DPI in PDF?

I'm in an unfortunate situation and desperate for some help. I'm a MAC user and I am editing a family history/autobiography book written in Word with hundreds of images. I am then converting to PDF to send to the printing company. I just found out that many of the images are below 300 dpi after the conversion to the PDF.  What are my options without starting over using all Adobe products? (My uncle is very sick with cancer and I need to get this book printed/bound.) I have all the settings in Word set to High Fidelity/Best Printing/Do Not Compress Images. I had my husband convert it on his Windows machine but same result.

-Is there another setting? Is this just a bad software combo (I know yes, but I am stuck). I am a newbie at all this.

-Change the resolution to at least 400 dpi or larger on each image before inserting into Word so that when it gets converted it should still be above 250/300?

-Can I batch update the resolution of the images from within Acrobat after all the editing is completely done? That would mean I couldn't go back and edit in Word once I do that.

Thanks in advance for any help you can provide.