The options I know have texts in searchable formats, but none is specific to major works or neatly organized like Gutenberg's Philosophy Bookshelf. You'll have to know the names and the authors you are looking for.
PhilPapers: They try to list all books and publications in academic philosophy journals. Not all of it is freely accessible, sometimes they just give a link to a paywalled source. But they have Our Archive, from which you can download.
PhilSci archive: This is specific to philosophy of science, but if they have it it is accessible in pdf or doc.
Internet Archive: Similar to Gutenberg, and not specific to philosophy. Mostly has old books (pre-1960), but in pretty much any format you want. Some books (especially newer ones) are only available for "borrowing".
Gallica: French analog of the previous one.
Philosophy Index: I'll mention it for completeness, but from the 20th century it only has Russell and Wittgenstein.