How should I handle memory management in .NET?

I have PDF documents with ~2 thousands pages. I tried to force GC.Collect() but this is a performance penalty for me

GC.Collect() is not necessary. You could simply call Dispose() (or Close()) on PDFDoc, PDFDraw, ElementBuilder, ElementWriter, TextExtractor, and other larger objects. All memory will be instantly released. You can also use IDispose pattern (on 'using' C# keyword) to immediately release these resources.