pdf or word of the article for the first submission I'm submitting an article to an Elsevier journal, but I'm hesitant about the file format I should choose as the first submission. According to this journal:


  Your Paper Your Way
  
  We now differentiate between the requirements for new and revised
  submissions. You may choose to submit your manuscript as a single Word
  or PDF file to be used in the refereeing process. Only when your paper
  is at the revision stage, will you be requested to put your paper in
  to a 'correct format' for acceptance and provide the items required
  for the publication of your article.


My field is computer networking.
The reason why I'm hesitant is that sometimes word files look different in other people's computer and the reason why I'm hesitant about submitting a pdf is that the quality of the images in pdf file is a bit lower than the word file.

I wanted to know which one is better for the first submission? pdf file or word file? or both of them in a zip?
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?