BigInteger

There are several (free) Pascal libraries for BigInteger (and sometimes Big Decimals like 1.23) functionality:

TForge

Homepage: https://github.com/Alexey-T/TForge-Pascal

Modern cryptographic library for Delphi and Lazarus/FPC implemented as a set of runtime packages. Project development was stopped, the last found snapshot v0.76 was put to GitHub.

Big Decimal Math

Homepage: https://benibela.de/sources_en.html#bigdecimalmath

This unit provides a BigDecimal-Type which stores arbitrary precision (BCD) decimal numbers and can be used like any native numeric type.