The forms on those two pages do not appear to be dynamic forms. That means that they are able to be opened in the various browsers you listed using the built-in PDF viewer.
From Form 1040 on irs.gov. (Similar results in the PDFs that I could open without a password.)
I'm going to presume your users are seeing a "Please wait..." message when they try to open the PDF. You can update to ES4 which will allow you to create your forms in HTML5 which will make the forms viewable by those users who are using other browsers. If you're stuck on an earlier version (like me), then you will need to make your forms static. You can still do calculations, but you can't add/remove fields and subforms.
(Radzmar provided a good explanation here about this situation. The post is referring to mobile devices, but the information still applies to those browsers you mentioned. Re: Unable to view the form in Mobile which was created using LiveCycle.)
Thanks jasotastic81 for your kind reply, it is very helpful! Yes, you are correct, users receive the ' Please wait ...' message and my form is a dynamic form with some features like users could auto add/delete table rows, etc. I tried to save it as a static form, then those features stopped working.
It seems upgrading to ES4 is the only way to go to bypass the browser issue.
Thanks again for the kind help!
I have a follow-up question here: Can ES4 HTML5 form be saved offline locally with the filled-in data, like the way regular XFA-based PDF does? Does ES4 HTML5 form support digital signature?
Thanks for your kind response in advance.