i have eight databases,this is the biggest one 
Am I right that this database contains basically just scanned books?
No.I have a lot of art history and Film History books, and they are very large
But are they books you scanned or downloaded from somewhere?
Same here with another discipline. There some books with several 100MB.
I sort my database for file size and backup the original PDFs to an external drive and a cloud drive in case that I need the excellent quality of the graphics – starting from the largest one.
In Devonthink, I only keep a medium quality PDF (which is easily less than 10% of the original and not really worse). Here I could store a link to the original or a tag to indicate that there is a larger file. But I am not organised enough and in case I feel that the quality of a certain PDF is not good, I just check whether there is a larger original. Keeps my unorganized mind sane.
I can imagine that you can save lots of space with this method, still having all PDFs available.
By the way, I also like to have everything in one database – I am one person, and all this is part of me and linked together, so I should not give up on the relations between everything.
recommend this tool ! https://avepdf.com/zh/hyper-compress-pdf
From the official website，like:
- springer Our business is publishing | Springer — International Publisher
- brill https://brill.com/
- Routledge https://www.routledge.com/
…
E-books of books published before 2000 tend to have very large file sizes. The scanning quality is often quite poor, with coarse images that take up a lot of space.
I have scanned versions of around 300 or more books.
Wow, 300 books scanned!
My Zotero library that is integrated in my main database contains of almost 1100 PDFs, but only a part of them are books. Scanned East Asian texts need a good resolution from scan, so it is not easy to go down with quality…
While this tool reduces the required disk space, it doesn’t affect the database’s properties (e.g its index and metadata) and therefore CPU & memory usage remain the same.
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And for more control over compression parameters, there’s PDFSqueezer, which Setapp users will already have.
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And non-Setapp users can purchase from the App Store 
