Error descriotion:

Solution:

you could edit apps/files_pdfviewer/js/files_pdfviewer-main.js and change

!==i[a]&&(c+="/"+encodeURIComponent(i[a]));return t&&(c=i[0]+"//"+i[2]+c),c}}

to

!==i[a]&&(c+="/"+i[a]);return t&&(c=i[0]+"//"+i[2]+c),c}}

REF:

https://github.com/nextcloud/files_pdfviewer/issues/381#issuecomment-845806364

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