The best format for any philatelic data is a SQL database. SQL can be run on any operating system, is portable, will never become obsolete, is free. SQL has been ubiquitous for the last 44 years and will be for the next 50 years (or more). SQL also supports multiple users adding and changing it in real time. Once you have a SQL database, you can use it as the foundation for a published PDF catalog, powerful interactive web page(s), or a set of album pages.
I am currently doing a Christmas Seals SQL database that I am targeting next month to release publicly. I will not have to constantly try to update a Word file or other application every time it needs to be changed. Simply add or change the underlying SQL database record (it is like changing a row in a Excel spreadsheet) and anything that is based upon it automatically updates itself.
But most importantly if I drop dead tomorrow others will easily be able to assume control and continue the work that I have invested. Since I have choosen to put the SQL database online, others will easily be able to add and update it with any operating system, any browser.
In terms of Seals value and demand, I have had 13 pounds of Seals on
ebay for sale; this is hundreds of sheets of both Easter and Christmas seals. After being listed for a week or two, the listing has not had even a single view. Demand is zero.
Don