Impossible to combine files including e-signatures/digital signatures

[Kia5EDB]

Hi, I can combine files without digital files, but I get an error message when trying to combine files including digital signatures, (see the picture below, file name §5.1) has an exclamation mark on the file and includes e-signatures.
I really need to combine all types of files in my job, even files including digital signatures.
I am using adobe pro dc, including in my creative cloud.  
Could you please help me solving this question. Thank you in advance. 

[margueritek · Accepted solution]

A digital signature prevents the file from being modified, since modification would invalidate the signature. So you can't "combine" files with digital signatures. What you can do is place the files in a portfolio within an enclosing PDF.

[Kia5EDB]

Hi and thank you for your answer. The thing is that my colleague could combine with other files (not correcting the file including digital signatures, just combining the different files together) and she has (if I am correct) Acrobat Pro 2017.
When I have contacted our IT support, they ask me to contact the Adobe support, since they say that Acrobat DC and Acrobat Pro shall be equivalent.
Do you mean that I should save all the PDFs as a portfolio instead?
Is is possible to show each PDFs bookmarks in a portfolio?
I usually send out an agendas with many included PDFs and would like the bookmarks to be visible for each file. 

Thank you in advance.