I have upgraded Sitecore 8.1 to 9.1 and now in 9.1 PDF items are not getting indexed and also they are not getting listed in a multilist with search field.
Sounds like you are missing the needed IFilter to index pdf content. Check https://doc.sitecore.com/developers/91/platform-administration-and-architecture/en/index-pdf-files.html for the information on 9.1:
The Sitecore Content Search API uses the native Microsoft Windows IFilter interface to extract the text content from media files for indexing.
However, to enable the Sitecore Content Search API to properly index the content in Adobe PDF files, you must install the Adobe PDF IFilter on every content management and content delivery server.
Currently, Sitecore only supports Adobe PDF IFilter version 9.
You can install Adobe PDF IFilter as a standalone IFilter or as part of Adobe Acrobat Reader. You can download Adobe PDF IFilter version 9 from ftp://ftp.adobe.com/pub/adobe/acrobat/win/9.x/.