@Agnius
Some progression, so now I save the attachment into a document library with PA cloud instead. I removed the Get attachment action in PAD since it is not working for me.

In PAD, I get the file content using SharePoint connector and then successfully convert binary data to file. Managed to merge the PDF files as well.

However, in "Get file content" when I change the hardcoded file name 31-test.pdf

to %AttachmentName%, it breaks. %AttachmentName% has the exact same value which is 31-test.pdf.

Get file content output value of the successful flow
"value": "%252fShared%2bDocuments%252fPlant%2bStreet%252fSigned%2b-%2bContract%2bof%2bSale%252f31%2b-%2bContract%2bof%2bSale%2b-%2b1%2 ...",
Get file content output value of the failed flow:
"value": "/Shared Documents/Plant Street/Signed - Contract of Sale/31-test.pdf",
I need the File Identifier to be dynamic