What Is the Best Way to Change Format Type?

What is the best way to change format type, for example, .PDF to .jpeg?

Yes but with conditions. I discovered today that Microsoft Word 2016 (may not work on earlier versions) is very capable of opening PDF documents. Right-click on the document and Open With… (Microsoft Word). This will only work where a typed document that has been converted/saved as a PDF. If your document was created from a scanned image then it won't be able to do so. Think of it as trying to change a photo of a page back into a page. However there is a work around. I do this sometimes, but it can be laborious if you need to do it for many pages but great for a few of them. This will require Google Photos. Take a screenshot or photo of your document using your smartphone, go into Google Photos and open the image you want. At the bottom of the open picture will be a round-edge square with a dot in it. This is Googles OCR (Optical Character Recognition) system; if you use this, it will recognise the text, which you can then copy to a document of your choice. Hope that helps.