If you are doing it manually, you can add Markdown syntax. For example, to convert a plain text header to a Markdown header, you can add # before the text for a top-level header. If you are using a tool, there are online converters and software like Pandoc. Pandoc is a command-line tool. For example, to convert a plain text file input.txt to a Markdown file output.md, you can use the command pandoc input.txt -o output.md.
Tools like Microsoft Word can import a Markdown file (by changing the file extension to .md and opening it in Word) and then save it as an RTF file. Another option is to use Pandoc with the command pandoc input.md -o output.rtf.
In Microsoft Word, you can go to the "File" menu and select "Save As" and choose PDF as the output format. Many operating systems also have built-in print-to-PDF functionality. For example, on Windows, when you print a Word document, you can select "Microsoft Print to PDF" as the printer and then save the document as a PDF.
Tools like Adobe Acrobat Pro can export a PDF to Excel. There are also online converters such as SmallPDF. When using Adobe Acrobat Pro, you can go to the "Export PDF" option and select Excel as the target format.
Image editing software like Adobe Photoshop can open a JPEG file and then save it as a PNG file. In GIMP (a free and open-source image editor), you can open the JPEG, go to the "File" menu, and select "Export As" and choose PNG as the format.
Tools like ImageMagick can be used. The command convert input.png output.gif can be used in the terminal. Some online image converters can also perform this conversion easily.