If you mean the Appendix then it's not secured and it should be possible to combine it with other (non-secured) files.
Are you trying to combine the file in Acrobat (if so, which version of it), or via the Create PDF online service?
What is the exact error message you're getting?
How did you create the other file you're using?
Thank you for your prompt response.
Using Acrobat DC V 2019.008.20081 I was trying to combine a genuine non-secured pdf file with a pdf that might have been created using 3rd party (CuteWriter) software (I can't recall - I used "print" from the web and I have two options for pdf). To be sure I have just now opened that questionable file in Acrobat DC and used "save as" to be sure it is now a genuine non-secured pdf file. I then tried "combine" again. This time it did not fully combine the files - but it did allow me to save the result as a "portfolio pdf". That is good enough for me. However, I was earlier under the impression that "combine" creates a normal pdf file - not a portfolio pdf - but that might just be my misunderstanding of how "combine" works.
I received no error message at any stage - which was my original complaint - if there is a problem (eg with the original file having been created by a 3rd party software) Adobe should be able to give me a warning. I have many hundreds of files created with CuteWriter and have always been worried that there could be some incompatibility with Adobe Acrobat's pdf format. About 10 years ago I received a large file from a major US company and Adobe Acrobat Reader said it was not a proper pdf file and it could not be opened. I have been worried about compatibility of pdf files ever since. Your question regarding what version I am using only exacerbates my original concern.