Maintaining Resume (document) integrity when exporting from Pages.

Apple does not guarantee the quality, or substance of the translation process when opening a Word document, or exporting to a Word document. Pages is not a Word clone and that matters a great deal.

If you want the best fidelity in a Word document opened by other non-Pages users, then use Word in the current Office 365, or Office 2019 for Mac. You will be working in the native document format instead of some translated approximation.

The LibreOffice Suite, though a MS Office Suite clone, still does not produce or save every nuance in Microsoft's Word application. That may or may not be a show stopper depending on the audience for your document.

I guess I would inquire what application that third party is using that reports a PDF is corrupt or inaccessible — unless they are using some brand-x PDF viewer that does not measure up to the latest Adobe Acrobat Reader DC. Check your exported PDF in that application, and if there are no issues, I would be suspicious of the recipient feedback.