OnSong uses the built-in viewer component from Apple to display PDF files. This issue often occurs with PDF's when your device is running low on working memory. This is especially apparent on older devices like the iPad 1 and iPad 2 that have less than 1 GB of RAM. Newer versions of iOS do not handle low memory issues when displaying PDF files, which results in the screen going blank.
To remedy this, please try going into the Utilities menu (gear icon) --> Settings --> Display Settings --> Song Viewer --> Renderer --> View Files As PDF --> Turn ON and tap Done.
This will use a more conservative method for displaying PDF files that may avoid showing blank pages due to memory issues.