It appears the OP has undergone an identity crisis…no matter…
OP:
Firstly, I encourage if not demand you make a complete backup copy of your entire original file set…Whew!, now nothing that follows is my fault! :-)
So, this could work, if I have a good understanding of the “spec” on what is needed:
Find what: (?s).+?\RSummary:[^\r\n]+\R(.+)
Replace with: \1
Search mode: Regular expression
Very high-level summary of what happens with the find and replace:
From the start of file (in a Replace in Files operation) match a string of any characters followed by a line-ending with “Summary:” immediately after; continue matching through (and including) the line-ending of the “Summary:” line. At that point, grab the contents of the remainder of the file and remember it. At replace time, substitute what was remembered (everything after the “Summary:” line) for the contents of the entire file. This effectively deletes the desired lines.
For the regex experts out there reading this, I DID try some things with the \A anchor (for start-of-file matching), but I recall reading something from @guy038 on a bug with \A in Notepad++'s regex engine, and my testing bore out that there is a problem with it.