My software environment:
Windows 10 Pro 1909, Build 18363.900 (64 bit)
Adobe Acrobat Pro DC Continuous Release 2020.009.20067
Microsoft Excel 2019 MSO (16.0.12827.20200) 32 bit

My Problem with Adobe Acrobat Pro DC:
Since years I combine MS-Excel-files in one PDF via right mouse-click and drop-down menu "Combine files in Acrobat ..." in Windows-Explorer.

Unfortunately, this procedure results nowadays in an error message: "Unable to open document" and a reading permission problem (see attached Screenshot).

There are no reading permission restrictions and I think it is a bug in this Acrobat-Release since it was in the past not an issue.

I have also attached two sample Excel-Files which are causing the described problem and hope the problem is a reproducible one and can be solved in future releases.

Kind Regards
Michael