Google Ranking Factor – Document Usability
Website design went through a phase of using a lot of Flash code to render cute little slide shows or animations for the user. Unfortunately (fortunately in my view) though, Flash code could not be penetrated by Google-bot to determine what it contained and if there was any rankable value therein.
Pages that were comprised solely of Flash code would bring zero SEO value, and negative value over the site as a whole. Flash is therefore not recommended.
HTML5 is a suitable alternative for modern websites. Flash is also not supported by some operating systems or browsers, so becomes unusable and un-viewable for some website users.
Other document types that Google cannot consider for rank are many, and these usually occur as attachments to pages, such as Word documents, Excel sheets, DWG computer aided design files and many many more.
If you have a lot of this type of content on your website, none of it contributes to rank, no matter what it contains.
Google-bot can however penetrate and index open PDF files which may contribute to rank. If your PDF file attachments are not optimised and may impact negatively on rank, seal the file so that text can not be scanned from the content. PDF documents are widely supported in web browsers and therefore are a very useful attachment document type for both users and SEO.