It is "external"
or "internal"
although internal (system files) is probably not useful here.
ContentResolver cr = context.getContentResolver();
Uri uri = MediaStore.Files.getContentUri("external");
// every column, although that is huge waste, you probably need
// BaseColumns.DATA (the path) only.
String[] projection = null;
// exclude media files, they would be here also.
String selection = MediaStore.Files.FileColumns.MEDIA_TYPE + "="
+ MediaStore.Files.FileColumns.MEDIA_TYPE_NONE;
String[] selectionArgs = null; // there is no ? in selection so null here
String sortOrder = null; // unordered
Cursor allNonMediaFiles = cr.query(uri, projection, selection, selectionArgs, sortOrder);
If you want .pdf
only you could check the mimetype
// only pdf
String selectionMimeType = MediaStore.Files.FileColumns.MIME_TYPE + "=?";
String mimeType = MimeTypeMap.getSingleton().getMimeTypeFromExtension("pdf");
String[] selectionArgsPdf = new String[]{ mimeType };
Cursor allPdfFiles = cr.query(uri, projection, selectionMimeType, selectionArgsPdf, sortOrder);
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