Acrobat feature for combining documents with digital signatures needs fixing

If printing, scanning, and merging is acceptable for an exhibit, then you can print the signed documents to PDF and merge them.  Printing to PDF can be automated. 

https://community.adobe.com/t5/acrobat-discussions/workaround-needed-for-quot-print-quot-action-limitation-in-action-wizard/m-p/14902635

This should save you time and paper.

Another idea in place of a portfolio is to attach all of the documents to one PDF with a clickable table of contents, or a list box, to open them.  This can literally be done in a minute:

1)  Drag all the PDFs into the attachments panel.

2)  Create a list box ("List Box1").

3)  Run the following script in the console:

var aray=this.dataObjects;
var aray2=[];
for(var i=0;i<aray.length;i++)
{
aray2.push(aray[i].name);
}
this.getField("List Box1").setItems(aray2);

Enter the following script in the Selection Change tab of the list box field:

this.exportDataObject({cName:event.changeEx,nLaunch:2});

As Bernd suggests, collecting PDFs into a Portfolio is the same as putting a bunch of signed paper documents into a folder.  So this is a solution. 

I think what you objecting too is the presentation. You'd like there to be a way to show the separate signed PDFs as if they were a continous document.  That would be a cool feature.

Perhaps another approach would be to create a summary/Index document with links to information in all the separate documents, so they are easy to navigate.  

Or perhaps a Legal document management system, like Case Map from Lexis Nexis. 

But I also agree that it would be useful to have a way to sign only the page content streams, so that the documents could be combined and annotated without breaking the signatures.  Take this one step further and there could be signed/certified sub-content, i.e., a portion of a page that's signed/certified. 

This however is not something Adobe can do directly. Creating this type of contained content certification would be a change in the PDF specification (ISO 32000), which can only be done by the keepers of the spec, The PDF Association. 

In Acrobat combine the files as portfolio.