Try changing

.BoostMatching(p => p.MatchTypeHierarchy(typeof(PdfFile)) & ((PdfFile)p).Name.Contains(searchString), 4)

to this

.BoostMatching(p => p.MatchTypeHierarchy(typeof(PdfFile)) & ((PdfFile)p).Name.Match(searchString), 4)

But that would only match PDF files where property Name matches the search query exactly, right? At least that was how I was taught Match works. I want to have PDF files where Name fully or partly contains the search query so if the query is "brev" then I want to boost PDF files with name "Skicka brev" etc.

Ah, you are right. Then I guess you have to add some kind of wildcard search extention.

Something similar to the FuzzyFilter method specified here:
https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2016/12/episerver-find---how-to-do-fuzzy-search/

I did a quick test: Document with Name "Sending letters" got match when using FuzzyFilter with "letters".

Hmm, do you mind showing how you use that filter? I saw that post but didn´t really think it would work together with the boosting, since it's not only that I want to use a fuzzy search in general, but actually boost those items.

I just did a quick test to verify that filter actually would match. I altered the extension to this:

public static ITypeSearch<T> FuzzyFilter<T>(
            this ITypeSearch<T> search,
            Expression<Func<T, string>> fieldSelector,
            string almost,
            double? minSimilarity = null)
        {
            var fieldName = search.Client.Conventions
                .FieldNameConvention
                .GetFieldNameForAnalyzed(fieldSelector);

            var wildcardQuery = new FuzzyQuery(fieldName, almost.ToLowerInvariant())
            {
                MinSimilarity = minSimilarity
            };

            //Add it to the search request body
            return new Search<T, WildcardQuery>(search, context =>
            {
                if (context.RequestBody.Query != null)
                {
                    var boolQuery = new BoolQuery();
                    boolQuery.Should.Add(context.RequestBody.Query);
                    boolQuery.Should.Add(wildcardQuery);
                    boolQuery.MinimumNumberShouldMatch = 1;
                    context.RequestBody.Query = boolQuery;
                }
                else
                {
                    context.RequestBody.Query = wildcardQuery;
                }
            });
        }

And then used it like this:

_client.Search<Document>()
                .FuzzyFilter(d => d.Name, query, 1)
                .GetResult();

That works alright, by itself as a separate filter to match the Name property against the query (which of course could be useful), but it´s not really working with boosting the way I want with PDF files specifically. However, I´m not sure as to where the requirement came from and the background so I´m going to dig a bit deeper into that to see if this could be a solution.

Thanks a bunch for this!