You are right in that I was little explicit. Let us talk about a private persons need and let us stay at the comparison with CDs as they were available in the 90ies. In another reply I derived today's needed disk size as 1-10TB. Let us also say that the access time should suffice to be able to watch a 4k resolved movie. You say archiving suggests durability and CDs may not have the same durability as tape storages. However I would say that a cheap price would compensate a possible lack of durability, because one could easily duplicate or restore important data.
In the mid 90ies there were 9GB hard drives availabe, which is roughly the same amount of data that fits on 14 CDs. Today we have 14TB sized HDDs, so an equivalent optical disk should be able to store 1TB of data. However today a private person easily has more than one device plus several external HDDs to store his or her data. So I guess it would not be too greedy to demand permanent media with a capacity of 10TB. Comparing with CDs the price could be 2USD. So in this comparison I honestly find a 6TB tape for 200USD incredibly expensive.
Okay I should have found out before that there are many good apps solving the problem. Thank you for your advices anyway! After testing some of them, let me also answer my question by myself. I tried the apps Simple Scan, Tiny Scanner, Cam Scanner and Microsoft Office Lens. I skipped Adobe Scan, as I had to register before being able to use it. (The Free versions of) Simple Scan and Cam Scanner had too much ads for my taste. I could not find Readdle's Scanner Pro for Android in Google's playstore.
Imo Tiny Scanner worked best for my particular problem. You can take picture after picture, cropping, stretching and brightness adjusting is almost automatic. After that you can save the scan series into a single PDF file.
Also of good quality is Open Note Scanner, which however does only cropping and brightness/contrast adjustment without stretching, cropping and assembling to a PDF file.
For photographs, paintings and screens I also liked Microsofts Lens, which is perfect for stretching and cropping without brightness/contrast and monochrome adjustment.
