Spliting PDF and get thumbnail while adding it to a ListView
This code leaves a bunch of open questions, minor ones:
- SplitPDF2(string input) what has been/happened to SplitPDF() or SplitPDF1()?
- what is lista2?  

Non-minor:  


Document/comment your source code. In the code.
Preparing thumbnails and splitting a PDF into separate-file-per-page do neither sound nor look related:
Do not put/implement both in a single procedure  
I can't seem to see a purpose in saving each thumbnail rendered into a MemoryStream ms that doesn't get used otherwise.  
You instantiate one and only one iTextSharp.text.pdf.PdfReader(input) for each input:
Do the same for PdfiumViewer.PdfDocument
it will add the thumbnail I don't see that.


Overall, I'm left mistrusting writing file-per-page PDF to be a useful step in accomplishing the larger task at hand.
wish someone could say where [to improve the] code so it doesn't take that long my crystal ball is dull, but profiling/timeit() might help.