Several ways to solve this:
1) You can use regex.
2) You can use IndexOf to give you first occurence of '_'. Then you use substring to find the next 8 characters.
3) You can use the Get column by row, column separator function. eg:
{STRING(GetColumn|1|2|_|xxxxxxxxxxxxxxx_20190123_xxxxxx.PDF)}
Row = 1. It uses '_' as column separator, and takes second occurence.
Edited by user
2019-01-24T11:59:44Z
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Reason: Not specified