PDF is a vector
DOC(X) is a structured document
The premise of PDF guaranteed format comes at the expense of editability.
You can understand it this way: the reason why Word documents cannot be guaranteed to be consistent after opening is that Word documents do not save the actual content of your document format, but save their corresponding codes. Different versions of software may have deviations in restoring these codes, which leads to differences in actual display effects. The advantage is that it is highly editable and can significantly reduce the size of the document.
The PDF document can be understood as directly saving the content in the document as a graphic. The graphics that can be saved as vectors are saved as vector graphics, and those that cannot be saved as bitmap graphics. Once saved as a graphic, the format is less likely to change, but many things are harder to re-edit, and the document size increases significantly.
As far as the doc format ([MS-DOC]: Word (.doc) Binary File Format) is concerned, there are certain differences in the reading operation between WPS and Word. This is still a difference at the bottom level, and in terms of the difference at the upper level, this is subject to the understanding of the WPS architects on the typesetting itself. This understanding is both technical in typesetting and terms of extra-technical balance (especially performance). Of course, there are also economic factors. WPS cannot be compared with Word in terms of code magnitude and development cost, and there is too much difference in size. So even if it is saved as a doc file, it is naturally impossible to guarantee the same on the two software platforms.
And why is the PDF guaranteed to be the same in most cases? This is because what WPS/Word reads is a document, and the file format itself does not specify what the final effect should be. The final effect depends entirely on the software itself.
The PDF format is designed to be directly page-oriented, and it can even be said that the format is inherently printer-oriented (and it is). PDF is also much simpler than doc/Docx in design, and it can even be understood that PDF is a vector graphics format.