Merging PDFs in Ubuntu Linux or Linux Mint was real easy using the pdfunite command in a terminal. To merge PDF files in Linux open a terminal window enter the following command:
pdfunite file1.pdf file2.pdf file3.pdf combined.pdf <enter>
where file1.pdf, file2.pdf and file3.pdf are the individual pdf files that will be merged and combined.pdf is the merged pdf file.
To merge PDFs in Linux from a GUI interface, try PDFSam from https://pdfsam.org/.