I have enabled lightning knowledge in our developer sandbox and we have content documents in the files related list. But we can't see the file preview and giving an error saying preview is not available, so that user need to download and see the file. Can I know the reason for this issue? I'm getting this issue only for large files with more than 3 pages.
Salesforce does not guarantee all file types and their contained features will properly generate previews in Content. Please refer this doc detailing the limitations of Content file previews.
Limitation around PDF files
Large numbers of pages. Exact number is dependent on their content but if the count is high (100+) it may cause the preview generation process to timeout. Note: The number of pages generally has more impact on preview generation than actual file size. Although the file may be under the recommended 25 MB limit, the process may not be able to generate a preview. A large number of shapes may cause the file to be too complex to render.
If a file is not generating a preview, or it's attributes are not displaying as expected in the online version, you may attempt to regenerate the preview. If regeneration is unsuccessful, upload an entirely new version of the file to see if the issue persists.