How to open a PDF form file that says Please wait...?

Try Master PDF Editor (proprietary). The free version is good for viewing, but for editing probably not (I read that it leaves watermarks).

You can also use the outdated, unsupported, probably very insecure, old Adobe Reader 9 for Linux.

Recent drafts of PDF next-generation (ISO 32000-2) note that there was a vote to deprecate XFA:

Editor’s note: XFA forms have been deprecated from ISO 32000-2 in accordance with the outcome of the letter ballot following the Pretoria meetings.

As explained at 3 Alternatives to the Adobe PDF Reader on Linux | Linux.com | The source for Linux information, features like Extended form filling (such as XFA Forms) and Embedded 3D models won't work in most open source implementations.

So I suggest pushing back to the people producing the documents, explaining that they are not accessible to people in a variety of situations, are not supported by e.g. the Chrome PDF viewer or open source viewers, and that they thus increase the attack surface and increase security risks. So they should drop use of XFA.