beli Your question is not clear. For example, are you creating PDF thumbnails for the PDF files? If you have 10.000 PDF files, it would take several hours to create thumbnails for all the files while the server will be at peak CPU. That wouldn't "crash the page during loading", but it would probably cause the server to become unresponsive while trying to scroll through preview images on the page. Even after a all PDF previews are created and cached, it will still be demanding for the browser when scrolling, because each image will be stored in memory. This would depend on the actual device.
If you aren't creating preview images for the PDF files, then you need not worry about image processing at least. The page wouldn't crash, but scrolling may get sluggish as soon as you hit several thousand files. The "sluggishness" will be proportional to how powerful the device is.
You need to provide more context, as the question is not clear. Did you already try? You definitely need to target reasonable file counts in each folder, but there is no specific limit, as it depends on many factors.