Thanks for reporting the issue. Do you encounter this issue with every PDF file or only certain type of large sized file.
Can you try editing the file and then save it and check if it freezes. Also it will be great if you can share a sample file with us.
Can you please provide the following details :
1. Does the app freezes when you try Fill and Sign with all PDF files or specific PDF files. If possible can you please share a sample PDF.
2. Did you try using Comments/Edit after opening the PDF?
3. Once the application freezes, open task manager and go to Acrobat.exe process, right click and create a process dump. Kindly share that with us.