Why pdf plays one moment than invalid format the next

Purchase a comic in pdf form than in couple of weeks I tried to view some of the comics most other volume is reading in valid format what a disaster so I purchased a thumb drive purchased the same comic transfer to thumb drive seemed alright few weeks later on a trip tried to play some of them volume 1 through 5, volume 2 and 3 now reads invalid format, it's like it's on a ticking time bomb,
Is there a software that format a invalid pdf format.

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