PDF Linearization, Optimization, Protection and Digital Signatures. PDFlib PLOP DS (Digital Signature) offers all features of PLOP, plus the ability to apply digital signatures to PDF documents. PDFlib PLOP is a versatile tool which can repair, linearize, optimize, encrypt, and decrypt PDF documents. PLOP linearization and optimization features create efficient and small PDF documents for fast Web delivery. PLOP protection features encrypt or decrypt PDF documents and apply or remove permission settings, such as »printing not allowed« or »content extraction not allowed«. The PLOP repair mode automatically detects damaged PDF documents and fixes the problems if possible. PLOP analysis features can be used to query arbitrary properties of a PDF document. In addition, document metadata can be retrieved and set.