Here's the solution that I can come up with:
SharePoint Document Library
Added a custom choice column, [SpecType], to match the user selection in the MS Forms.
For example: Marketing Spec1, Marketing Spec 2, Sales Spec 1, Sales Spec2 etc.
Same as mentioned in the requirements, that is, user can choose one or more specs.
Screenshots showing the flow with additional details and explanation
The goal is to capture the "Select Documents" options from the MS Form Response and put them into an array to retrieve specs/docs from the SP library. Having analyzed the output of [Select Documents], which is an MS Form field, found that it's structure in the Flow output looks like
["Marketing Spec 1","Marketing Spec 2"] , so need to replace few chars and split the string to convert into an array and capture it in a [Compose] action. Please note that
outputs('Get_response_details')?['body/rfa0aef609e874b14a40f4f1674a08044'] needs to be selected from the dynamic content as the GUID value might be different in your case.
split(replace(replace(replace(string(outputs('Get_response_details')?['body/rfa0aef609e874b14a40f4f1674a08044']), '[', ''), ']',''), '"',''), ',')
Store the output from [Compose] in [ArraySpec]
Within the [Apply to each], need to retrieve file content based on the [Identifier] property and store into array variables to used in sending emails
Could not find an easy way to zip the attachments, so use multiple send email constructs using [switch control]. This is something to be enhanced.
Expression - 2
To send the email, get the file content and file name from the array variables
Expression - 3
when two docs/specs are requested.