You cannot convert PDF to HTML file format. However HTML can be converted via some JS and jQuery
You can't convert a HTML into CSS in the same way you can convert, say a .wmv file to a .mpg file. CSS is a way to style HTML items but doesn't use the same language as HTML. There may be software solutions that may be able to extract the styling instructions written in HTML and convert the language into CSS but failing that you'll have to rewrite the HTML styling into CSS manually. Whilst you can have CSS and HTML in the same .html document generally the CSS will be the styling and the HTML will be the content. I say 'generally' because it is possible to add 'content' using CSS and to still style using HTML within the same document.
You cannot convert an image to a markup file.
There are many tutorials online which will teach you how to convert a PSD file to an HTML file. There are also automatic converters you can purchase that will make the task much less onerous.
You can download a picture in an HTML page to any format. The download="file-name.jpg" can be used to convert it.
ASP.NET is not a programming language. However to come back to your question, you can use a label control to write the description. I prefer to use HTML file rather than an ASPX file for such requirements.
you open a HTML file using your internet browser
An HTML file is being printed to the screen by the browser. To convert what is on the screen you can take a screen shot. Some sites have the ability to save the page as PDF which is basically an image file like a JPG. The PDF file can then be saved as a JPG.
When the HTML file is located locally, you can open the HTML file to Word and then save it as a Word file. However, as webpages are made to fit a screen instead of on paper, the site may not look as desired when saving it as Word document.
.html is not an image file, so it can't be converted to .jpg unless the .html file has images in it, in which case you would show extensions and then click in the name box and change .html to .jpg