I have investigated elcom's software what it used to say is that pdf files that can be opened, but use password protection to prevent changes are pretty easy to circumvent. PDF files that require a password to open are more difficult for them to open, they use dictionary attacks and when that fails it uses brute force methods. Brute force methods are pretty successful with weak passwords for the most up to date security that Adobe offers. If you pick a truly random 64-bit security password, it will take some time to crack it.
As to your ability to open a secured password in 20 years from now? Heck there is no guarantee that your non-secured pdf file will be readable in 20 years. With PDF/A your chances are pretty good as it uses standards based file format, but there is no guarentee that Adobe or any other company doing business today will be in business 20 years from now.
In general, there are no guarentees today's data will be usable 20 years from now. To be sure you can acccess needed files, you first have to make sure that every year (or five years-you decide) that you can open every needed file and its backups to be sure the file hasn't degraded. The only sure thing is that data and information degrades. You then have to ensure you have access to programs that will open the files twenty years from now. I sure woudn't want to be responsible for maintaining Windows 7 computers and their hardware to be usable twenty years from now. So to maintain usefulness, file formats and/or encryption-methods may need to be updated over the years.
I have files from more than 25 years ago. Keeping the files usable is arduous for an individual. For a company well ...
