Some primary source links

• For Euler stuff go to the Euler Archive.
• The Jahrbuch Project covers
  • A literature database based on the "Jahrbuch über die Fortschritte der Mathematik" (1868-1942)
  • An archive of digitised mathematical publications at the Staats- und Universitätsbibliothek Göttingen
• It is unclear if Göttinger Digitalisierungszentrum: GDZ is properly contained in the above. Here they have scanned 3231 mathematical works.
• The Univeristy of Michigan Historical Mathematics Collection has digitized 991 math books (no journals). Try here for "collected works" of someone.
• Similarly, Cornell University Library Historical Monographs Collection has 456 Monographs and 512 math books scanned in.
• The Czech Digital Mathematics Library: DML-CZ
• NUMDAM is entirely in French. Good luck.
• As is Gallica
• The World Digital Mathematics Library :WDML lists several more sources, though primarily more modern (1920-present).