There are several reasons that could cause this issue. Please try to check the following steps to fix the issue.
Limitations to Users with Free Membership
The free version of PDF Reader only enables you to store 10 files. As a temporary solution, please try to remove unwanted files from PDF Reader when you store more than 10 files or upgrade to the premium membership.
The App doesn't Support the File Format
PDF Reader supports mainstream file formats (see the formats listed below). If you import the file that PDF Reader doesn't support, the file won't be able to be opened.
- PDF format: PDF, vitrium protected PDF
- Ebook format: epub
- Comic book format: cbr, cbz
- MS Office format: doc, docx, ppt, pptx, xls
- iWork format: pages, key, numbers
- Image format: png, jpg, thm, jpeg, tif, tiff, gif, bmp
- Compressed format: zip, rar,
- Audio format: aiff, aifc, wav, mp3
- Video format: mov, mp4, mpv
- Other format: txt, html, winmail.dat, Safari webarchives, and more
The Files are Broken
If the file has already broken before you saved it to the PDF Reader app, the file can not be opened, either.