pdf or word of the article for the first submission
I recommend you submit PDF, for two reasons:


Only PDF gives you full control over the formatting of the document as received by peer reviewers;
Those who are accustomed to typesetting in LaTeX (of which I suspect there are a fair few in your field) may be biased against authors using Word. Admittedly a Word document with default formatting settings could not be mistaken for a regular LaTeX document, if only because of the characteristic typeface used by the latter, but a well designed document might almost be mistaken for a XeLaTeX-generated PDF. At any rate no one will be triggered from the production-neutral process of opening a PDF file, but some may resent having to deal with .docx.


The PDF image quality is a concern. Have you set image compression in the PDF export dialog to prioritize quality?