FLEXIBLE FILM BASED ON FLUORINATED POLYMER [Field of the invention] The present invention relates to a PVDF-based film for protecting substrates and to the substrates covered with this film. It also relates to a multilayer structure combining a PET or PEN sheet with the PVDF¬based film. Finally, the invention relates to the uses of the PVDF-based film or of the multilayer film, especially for the protection of photovoltaic modules. (Technical problem] PVDF (polyvinylidene fluoride), because of its very good weatherability and resistance to radiation and chemicals, is a polymer useful for protecting objects and materials. It is also appreciated for its shiny appearance and its graffiti resistance. It is therefore common practice to coat all kinds of objects with a PVDF film. However, the film must have good adhesion to the substrate to be protected and exhibit very good thermal resistance as required in external applications exposed to severe weather conditions or to conversion processes carried out at high temperature. It is also necessary for the film to exhibit good flexibility and a high strength so as to withstand mechanical stresses that arise when the film is being placed over the object, or once the film has been placed on the object when the latter is formed, for example by drawing. One applicative test used consists in tearing a film that has undergone oven ageing and in seeing whether the tear propagates easily or not. The Applicant has developed a PVDF—based film which exhibits good flexibility, has a high strength and can be used in certain applications. It has also developed a multilayer structure by combining the PVDF-based film with a PET or PEN sheet. (Prior art] Application EP 1 382 640 describes a film consisting of two or three layers based on a PVDF homopolymer or copolymer. The PVDF copolymer contains 0 to 50% comonomer. The examples describe the use of PVDF homopolymer. Application EP 1 566 408 describes a film consisting of two or three layers based on PVDF homopolymer or copolymer. The PVDF copolymer contains 0 to 50%, advantageously 0 to 25% and preferably 0 to 15% comonomer. The film contains no filler. Application EP 172 864 describes a photovoltaic cell protected by a PVDF/PET film. There is no adhesive layer between the PVDF and the PET. International Application WO 2005/081859 describes multilayer films based on fluoropolymer and acrylic polymer. Patent US 6 555 190 describes a multilayer structure comprising, in order, a PEN layer, an adhesive layer and a layer of a fluoropolymer (PCTFE, PVDF, etc.) The adhesive layer comprises a polyolefin functionalized by an unsaturated carboxylic acid or anhydride, or else a homopolymer or copolymer comprising, as monomer(s) acrylic acid, acrylates and alkyl acrylates, which is optionally modified by an unsaturated acid or anhydride. Application US 2005/0268961 describes a photovoltaic module protected by a film comprising two fluoropolymer layers, one having a melting point above 1350C and the other having a melting point below 1350C. Application US 2005/0172997 or patent US 6 369 316 describe a photovoltaic module protected by a polyvinylidine (TEDLAR) film. (Figures] Figure 1 shows an assembly 1 comprising a photovoltaic module 2 protected by 3. Figure 2 shows schematically an assembly 4 comprising a photovoltaic module 2 protected by the film 3. The module 2 comprises a layer 6 consisting of photovoltaic cells encapsulated in a thermoplastic resin (e.g. EVA) The module 2 also comprises a glass plate 5 serving as front support for the module 2. The protection is provided by a multilayer structure 3 comprising a PET sheet S olaced between two PVDF—based films, references 7 arid 7'. Figure 3 shows a deep—drawn part 9 (in the schematic form of a cup) made of a steel sheet 10 protected by a PVDF-based film, reference 11. (Brief description of the invention] The invention relates to a multilayer structure using a PVDF-based film as defined in claim 1. The invention also relates to the uses of said structure and to the process for manufacturing a PVDF— based film. (Detailed description] The term "PVDF" denotes PVDF polymers, namely vinylidene fluoride (VDF, or CH2 = CF2) homopolymers and VDF copolymers preferably containing 50% VDF by weight and at least one other fluoromonomer copolymerizable with VDF. Preferably, the PVDF contains, by weight, at least 50%, more preferably at least 75% and better still at least 85% VDF. Preferably, to increase the flexibility of the film, it is possible to use, for any one of the layers of the film, a PVDF advantageously comprising, by weight, 5 to 20%, advantageously 7 to 13% of at least one fluorinated comonomer per 80 to 95% and advantageously 87 to 93% VDF (this type of PVDF will be referred to hereafter as "flexible PVDF") . Preferably flexible PVDE is used for compositions A and B. Advantageously, the fluorinated comonomer copolymerizable with VDF is chosen from: vinyl fluoride; trifluoroethylene (VF3); chlorotri¬ fluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TEE); hexafluoropropylene (HEP); perfluoro(alkyl vinyi~ ethers, such as perfluoro(methyl vinyl) ether (Pt4VE, ~erfIuoro(ethyl vinyl) ether (PEVE) and perfluoro