It's said that only a few types of files are supported on Kindle including .azw, .azw1, .txt, .mobi, and .prc. However some of my .DOC files are still shown on Kindle, while other .DOC files are not. Wondering why? Are there some specifications to adjust to make it happen?
You are mistaken. If you upload a .doc to Amazon via Personal Documents Service, it should convert automatically -- no problem! https://www.amazon.com/gp/help/customer/display.html/ref=hp_pdoc_main_short_us?nodeId=200767340
Now it may not look great, but generally you are mainly interested in the text.
Apart from idiotprogrammer's answer, there are many methods that you can send the doc file to Kindle. You can convert it to PDF or use one of online doc to mobi convertors.
Another interesting approach to have the best result is to following this steps:
Import doc to google drive (go to https://drive.google.com/drive/ and click on "new" button and select file upload)
Open imported doc. You may fix some small problems in the file. then select "file" menu and select "publish to the web..."
In chrome, Copy the address of published page and paste it in new page.
Use "send to kindle" extension to send your published page into the Kindle. If you want, you can now return to google doc and stop publishing your document.