Tested with JabRef 4.21 64 bit Windows

Steps to reproduce the behavior:

1. Create new BibTeX library
2. Import into current library
3. Select PDFContent and choose pdf file.
4. Select Open
   file is opened
   //import second pdf file
5. Import into current library
6. Select PDFContent and choose pdf file.
7. Select Open
   file is not opened
   status. importing in unknown format...

Uncaught exception occurred in Thread[JavaFX Application Thread,5,main]
java.lang.ArrayIndexOutOfBoundsException: -1
at org.jabref.logic.importer.fileformat.PdfContentImporter.proceedToNextNonEmptyLine(PdfContentImporter.java:525)
at org.jabref.logic.importer.fileformat.PdfContentImporter.importDatabase(PdfContentImporter.java:228)
at org.jabref.gui.importer.ImportMenuItem$MyWorker.run(ImportMenuItem.java:128)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at spin.Invocation.evaluate(Invocation.java:175)
at spin.off.SpinOffEvaluator.evaluate(SpinOffEvaluator.java:120)
at spin.ProxyFactory.evaluteInvocation(ProxyFactory.java:85)
at spin.JDKProxyFactory$SpinInvocationHandler.invoke(JDKProxyFactory.java:102)
at com.sun.proxy.$Proxy30.run(Unknown Source)
at org.jabref.gui.importer.ImportMenuItem.automatedImport(ImportMenuItem.java:78)
at org.jabref.gui.importer.ImportFormats$1ImportAction.doImport(ImportFormats.java:88)
at org.jabref.gui.importer.ImportFormats$1ImportAction.lambda$actionPerformed$0(ImportFormats.java:75)
at java.util.Optional.ifPresent(Unknown Source)
at org.jabref.gui.importer.ImportFormats$1ImportAction.lambda$actionPerformed$1(ImportFormats.java:75)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
at java.lang.Thread.run(Unknown Source)