You can do this in most editing software (word processors/text editors) that lets you search/replace paragraph marks or line breaks. Here's is an example strategy described in Word syntax where
^p is a paragraph break, assuming no distinction between new-line characters and page breaks (which is the case for text files), and in which I will use _ for space characters.
First insert extra paragraph marks after sentence punctuation:
Remove leading and trailing spaces, keep repeating until no more found:
Reduce the number of consecutive paragraph breaks to two at the most. Keep repeating until no more found:
Additional steps if you want to stitch sentences together into one parapraph:
$$##$$ (or any string that is not in your document)
- Put all this in a keyboard macro if you need to do it in multiple documents.
- You may wish to reduce to one paragraph break only. Then you need a variation of this strategy.
- You can use regular expressions as well, but I'm not fluent with them.
- For that additional step your software must be able to handle very long lines, because you will have an intermediate result which is basically just one line.
- Do all this on copies of your files so that you have a backup.