how can I convert my kindle books (located on my computer) from pdf form to mobi so I can send them to my kindle?
It is rather easy with the software Calibre. https://calibre-ebook.com/download
Check it out, just download the tool to your PC, go into it and load the eBooks you want to put onto your Kindle in Mobi.
I personally use this thing to convert my PDFs, RTFs and EPUBs to MOBI.
Then you can simply connect your kindle to your computer via USB and transfer the files that way (just drag and drop them or copy and paste them) or you can even email them to yourself (free of charge) using your kindle email address.
There are times where the metadata of your ebook (things such as author and title) are in a format that you're not happy with, for example they may have underscores instead of spaces and you wish to change this. I recommend using this.
It depends on what type of PDF file you are talking about. You can usually convert PDF back to Word or Latex and then convert it to MOBI or AZW and AZW3 to have the best possible result. For example if the pdf made in Latex, you can follow this solution.