Locking Images on MS Word Doc

What happens if you create the document in Pages and export directly to PDF from there?
Image positioning and text reflow is a nightmare in any version of Word I've used, and changes in font rendering across platforms doesn't exactly help.

 

OP, IMO you're never going to see your document display identically on two computers, in Windows or macOS. I've been using Word on both OS platforms since 1989 - be sure you're you're using a template that all parties have (not the Normal Template) with embedded styles (with the "Automatically Update Document Styles" option enabled), same fonts from the same source, same printer drivers/PPDs with identical layouts. Plus, Macs and Windows display text differently, going back to that 96dpi vs 72dpi thing that started back in the 90s. Text-based PDFs will have similar issues. I'm lining this out since IMO you used the term "lock" and not "anchor" as it's termed in Word...

The only options you have are to either send a PDF file with each page rendered as an image-based format (PNG or JPEG are what I use when I send a document to clients) or, if you insist on sending a Word file or text-based PDF and you're using Windows, embed the font(s) you're using in your document and use the appropriate Anchor for your purposes; your document likely will never appear to others as it does to you if you choose text-based options. You didn't provide the version of Word that you're using - every version of Word for Windows since 2003 has a different means of anchoring an object with differing options for your intended purpose and I don't have time to line them out for you. Look in your app's Help...