Hello @Emanuela L.
I hope you are doing well, and thanks for reaching out.
Please note that Acrobat's built-in Split does not support splitting by a keyword or text string. Its only split criteria are number of pages, maximum file size, or top-level bookmarks. So there is no native "split wherever this text appears" option.
Suggestions:
If each payslip is a fixed number of pages
If every payslip is, say, 1 page (or a consistent N pages), you do not need keywords at all. Use Tools > Organize Pages > Split, set Split by "Number of pages" to 1, and Acrobat produces one file per payslip. The only gap is naming, page-count split names files sequentially, so you would rename afterwards. For standard single-page payslips, this is usually the simplest, most reliable solution.
If the PDF has a top-level bookmark at the start of each payslip named with the employee name/ID, then Split by "Top-level bookmarks" and, in Output Options, tick "Use bookmark names for file names." That both splits and names each file per employee. The catch is that Acrobat will not auto-create a bookmark at each keyword occurrence, so this only avoids scripting if your payroll export can already produce a bookmark per employee.
Acrobat JavaScript (Acrobat Pro): a script uses getPageNthWord to find the identifier and extractPages to write each range to a new file named from that identifier. This does exactly your use case, but it is custom scripting, not a built-in button, and needs Acrobat Pro.
For more details, please refer to these help articles:
https://adobe.ly/4ccmgln
https://adobe.ly/4ccmhpr
https://adobe.ly/4wWkdcx
https://adobe.ly/4wWke05
I hope this helps, and please reach out if you need any assistance.
Regards,
Anand Sri.