Heres a collection of Bitcoin whitepapers and documents, all in PDF.
Bitcoin:
https://bitcoin.org/bitcoin.pdf
Lightning Network:
https://lightning.network/lightning-network-paper.pdf
Coinpool:
https://coinpool.dev/v0.1.pdf
Colored Coins:
https://www.etoro.com/wp-content/uploads/2022/03/Colored-Coins-white-paper-Digital-Assets.pdf
Pegged Sidechains:
https://blockstream.com/sidechains.pdf
Proof-of-Work Sidechains:
https://eprint.iacr.org/2018/1048.pdf
Tradelayer:
https://tradelayer.org/whitepaper.pdf
Simplicity:
https://blockstream.com/simplicity.pdf
Discreet Log Contracts:
https://adiabat.github.io/dlc.pdf
Merged Mining:
https://eprint.iacr.org/2017/791.pdf
If anyone else knows of any (relating to bitcoin and its various sidechains/L2's, etc ) that should be here, comment below!