feat: add pdf display behind doc validation and an issue fixed

• There are 2 functions pdf_display: one for the display of the pdf behind the document validation and another one for the modification of titles
• close #1 (closed) : it's possible to completly remove a pdf when all documents
• close #7 (closed): change level of title for choosing is possible
• close #12 (closed) : change pages when checking titles is possible
• the final merging seem to be functional in every weird cases