I found a site with 100 books in Spanish that are public domain. I downloaded the pdf to my computer of one of the books and it imported only part of it. Only about 28 pages of think about 250 that are in the book.
This may be a link to it:
DeTierraLuna.pdf - Google Drive
I think the problem you’re running into is the formatting in the PDF, not the length of it.
If you have Micrsoft Word, you can import the PDF and then copy the text for each chapter into the lesson in LingQ.
Myself, I’m rather wary of using 19th century reading material. Sure, they’re free of copyright, but the words are usually not the same as used today. So you can be reading esoteric vocabulary that nobody speaks in today’s world.
Epubs work better. I imported an epub with over 1 million words (I didn´t even realise).
I have exactly the same problem with importing longer YouTube videos. I found a 2 hour long podcast and the lesson only ended up including about 40 minutes worth.
There is a limit I think this is around 40-45 minutes in length. You can’t import very long videos.
with Calibre ebook conversion software (free to download and set up)you can simply convert to txt format and delete all the irrelevant parts such as extra information related to author’s books, quotes, bibliography and so on, I never had issues with importing ebooks with the txt format.