Many people will tell you that Portable Document Format (PDF) is an open standard and could/should be used for the long term storage of your documents. It's true. The International Organization for Standardization (ISO) standardized it in ISO 32000. PDF was originally proprietary to Adobe, but became an open standard in 2008. It is royalty-free and can be implemented by any company or developer.

But this doesn't mean that PDF is the best (or a good) way of storing your documents or displaying them on your web pages. A PDF is a standard way of integrating the presentation of data with the data itself. There are a few problems with this:

The presentation format is typically fixed. Most PDF documents are electronic versions of 8.5x11 inch pieces of paper. If visitors to your website are like mine, many or most of them are using small-screen smartphones, such as Apple's iPhone. How does a PDF file display on an iPhone?

Very poorly. The text if often way too small to read. A PDF does not adapt to the physical computer (or phone or tablet) screen well.

A better way

XML is a standard for archiving the abstract data/information.

You can go from information to presentation easily. Retrieving information from the presentation is much more difficult and may be error prone.

If you're looking to preserve your documents for a long term, such as 100 years, you need to choose a data format that is not proprietary or based on products or tools that intermix the data's presentaion with its storage format. The eXtensible Markup Language is such a data format.

Recommendations

Converting your PDF to HTML is a great way to adapt to physical computer screens. The problem is, if you don't have access to the program that originally created the PDF, you may find it very difficult to convert to another format, like HTML.