Using VB.NET PDF SDK
VB.NET PDF Library: how to crop pdf pages using visual basic .net

Dim inputFilePath As String = "C:\1.pdf"

' Open file And get the target page
Dim doc As PDFDocument = New PDFDocument(inputFilePath)
Dim page As PDFPage = doc.GetPage(0)

' Get media box of the target page.
Dim bbox As Single() = page.GetPageBoundary(PDFPageBoundaryType.MediaBox)
Console.WriteLine("Media Box: llx={0}, lly={1}, urx={2}, ury={3}", bbox(0), bbox(1), bbox(2), bbox(3))
' Get crop box of the target page.
bbox = page.GetPageBoundary(PDFPageBoundaryType.CropBox)
If bbox IsNot Nothing Then
    Console.WriteLine("Crop Box: llx={0}, lly={1}, urx={2}, ury={3}", bbox(0), bbox(1), bbox(2), bbox(3))
Else
    Console.WriteLine("No Crop Box")
End If
' Get bleed box of the target page.
bbox = page.GetPageBoundary(PDFPageBoundaryType.BleedBox)
If bbox IsNot Nothing Then
    Console.WriteLine("Bleed Box: llx={0}, lly={1}, urx={2}, ury={3}", bbox(0), bbox(1), bbox(2), bbox(3))
Else
    Console.WriteLine("No Bleed Box")
End If
' Get trim box of the target page.
bbox = page.GetPageBoundary(PDFPageBoundaryType.TrimBox)
If bbox IsNot Nothing Then
    Console.WriteLine("Trim Box: llx={0}, lly={1}, urx={2}, ury={3}", bbox(0), bbox(1), bbox(2), bbox(3))
Else
    Console.WriteLine("No Trim Box")
End If
' Get art box of the target page.
bbox = page.GetPageBoundary(PDFPageBoundaryType.ArtBox)
If bbox IsNot Nothing Then
    Console.WriteLine("Art Box: llx={0}, lly={1}, urx={2}, ury={3}", bbox(0), bbox(1), bbox(2), bbox(3))
Else
    Console.WriteLine("No Art Box")
End If

Dim inputFilePath As String = "C:\1.pdf"
Dim outputFilePath As String = "C:\output.pdf"

' Open file And get the target page
Dim doc As PDFDocument = New PDFDocument(inputFilePath)
Dim page As PDFPage = doc.GetPage(0)

Dim bbox As Single() = page.GetPageBoundary(PDFPageBoundaryType.MediaBox)
' Add (Or update) bleed box.
page.SetPageBoundary(PDFPageBoundaryType.BleedBox, bbox(0) + 20, bbox(1) + 20, bbox(2) - 50, bbox(3) - 60)
' Add (Or update) trim box.
page.SetPageBoundary(PDFPageBoundaryType.TrimBox, bbox(0) + 60, bbox(1) + 40, bbox(2) - 100, bbox(3) - 160)
' Add (Or update) art box.
page.SetPageBoundary(PDFPageBoundaryType.ArtBox, bbox(0) + 90, bbox(1) + 50, bbox(2) - 150, bbox(3) - 260)

' Save file
doc.Save(outputFilePath)