If you do not have the pdftk installed, follow the below steps:
For(Ubuntu/Debian)
sudo apt update
sudo apt install pdftk
For(CentOS/Fedora), follow the instructions in the link below to install it:
https://www.pdflabs.com/docs/install-pdftk-on-redhat-or-centos/
Once installed, use the below command to split a single multi-paged PDF file to individual PDF files with each page
$pdftk your_multi-page_file.pdf burst
This will split each page from multi-page PDF file to each individual PDF file.
Similarly, to merge few selected PDF files in to one PDF file, use the following command:
$pdftk file1.pdf file2.pdf cat output mergedfile.pdf
If you want to merge all the PDF files in a specified directory, you may use the wild card character(*) as follows:
$pdftk *.pdf cat output mergedfile.pdf
‘*’ implies all the files with .pdf extension
You may find few more tips on dealing with PDF files here