Try it maybe in this way:
    Dim AppName As String, WrkOrdr As String, Location As String, PDFName As String, curdrv As String, curpth As String
    Dim AppDate As Date, AppTime As Date
    Dim ws As Worksheet '! if it has not been declared before
    
    Const strPath As String = "H:\PK\Spraying and Fertilizing\Chemical Application Records\"
    
    Application.ScreenUpdating = False
    
    '"ws" should be declared before, by "Set", is it declared ?
    'if not, then e.g.:
    Set ws = ThisWorkbook.ActiveSheet
    
    'Get values from form fields to generate dynamic file name
    AppName = Trim(ws.Range("H9").Value)
    WrkOrdr = Trim(ws.Range("L10").Value)
    Location = Trim(ws.Range("H13").Value)
    AppDate = Trim(ws.Range("G35").Value)
    AppTime = Trim(ws.Range("H35").Value)
    
    'Generate file name string
    PDFName = "ChemApp_" & AppName & "_" & WrkOrdr & "_" & Location & "_" & Format(AppDate & " " & AppTime, "m_d_yyyy_h_mm AM/PM") & ".pdf"
    
    'Possible but not needed - Part A
    'curdrv = Left(CurDir, 1)
    'curpth = ThisWorkbook.Path
    'ChDrive Left(strPath, 1)
    'ChDir strPath
    
    ws.ExportAsFixedFormat Type:=xlTypePDF, _
                            Filename:=strPath & PDFName, _
                            Quality:=xlQualityStandard, _
                            IncludeDocProperties:=True, _
                            IgnorePrintAreas:=False, _
                            OpenAfterPublish:=True
    
    'Possible but not needed - Part B
    'ChDrive curdrv
    'ChDir curpth
    
    Set ws = Nothing
    
    Application.ScreenUpdating = True
Ps.:
1) use "&" to concatenate strings, not "+"
2) The 'ChDir' statement changes the default directory but not the default drive