Thank you, chas_m,
1. does not apply. I did not do it and nobody else has access to my Ipad.
2. this problem only appeared in Pages. But I realize it is only pages where different fonts are relevant, so this is possible.
3. the problem appeared in original documents, not after copying.
If this is a hint that the disc is starting to fail - only after less than two years after purchase that is a serious problem. I wonder if this is a reason to try go for my three years applecare guarantee?
The strange thing is, that these characters appeared suddenly not connected to any operations I actively did on the documents. I opened the table with my travel plan e.g., i.e. dates, cities, trains or hotels and suddenly there were these letters or numbers in a different font - additions to, not conversions of my own writing, sometimes between words, sometimes ripping words apart by appearing right in the middle of a word. And since they were of a different size, sometimes bold, the whole document looked very strange.