Is there an online PDF shop manual for a 1979 W116 300 SD available for download? I can't seem to fine a hard copy anywhere.

If you're looking for Chassis & Body Manual, they are available on E-Bay.
https://www.ebay.com/itm/MERCEDES-SHOP-MANUAL-SERVICE-REPAIR-BOOK-116-CHASSIS-BODY-WORKSHOP-450SEL-300SD/233256141956?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D225640%26meid%3Db94226 ca76024250b6ff7daae6a2b88e%26pid%3D100005%26rk%3D3%26rkt%3D12%26mehot%3Dco%26sd%3D193290817975%26itm %3D233256141956%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithTitleNsfwFilter&_trksid=p2047675.c100005.m1851
(I know it's for the 420SEL but the chassis will be the same.)
If you need Engine Manuals, the W126 manual should get you very close. The difference might be transmission control.
http://www.peachparts.com/shopforum/general-information/248948-mercedes-service-manual-source-cd-dvd-paper.html

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Sam

84 300SD 350K+ miles ( Blue Belle )

Mercedes-Benz Model 116

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022