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How to convert xls file to xlsx file using C#?

I was developing an application which read data from an excel file, but when I try to open it, an exception  was thrown if the source file is saved with the xls format (File contains corrupted data error when opening Excel sheet with OpenXML). indeed when I save this file with the xlsx format it works fine. please help me to solve this problem.

Use Free Spire.XLS dll available via NuGet.
Sample:
Workbook workbook = new Workbook();
workbook.LoadFromFile("Input.xls");
workbook.SaveToFile("Output.xlsx", ExcelVersion.Version2013);

For reliably reading XLS files you could use ExcelDataReader which is a lightweight and fast library written in C# for reading Microsoft Excel files. It supports the import of Excel files all the way back to version 2.0 of Excel (released in 1987!)
Alternatively you could use a file conversion API like Zamzar. This service has been around for 10+ years, and provides a simple REST API for file conversion - it supports XLS to XLSX conversion. You can use it in C# and it has extra features like allowing you to import and export files to and from Amazon S3, FTP servers etc.
Full disclosure: I'm the lead developer for the Zamzar API.

You cannot read xls files with OpenXML.
The solution from Microsoft is to read the xls file with Office Interop (but Interop is not recommended to be used on the server), transfer data from Interop step by step to OpenXML.
Another solution is to use an Excel library like EasyXLS and convert between these two Excel file formats:
ExcelDocument workbook = new ExcelDocument();
workbook.easy_LoadXLSFile("Excel.xls");
workbook.easy_WriteXLSXFile("Excel.xlsx");
Find more information about converting xls to xlsx.
I am not quite sure why you need to convert the file and why you don't just read the xls file, using a different technology then OpenXML, for sure.

XLS is the older Excel file format. XSLX is the newer format stored as OpenXML. XSLX is actually a zip file with the various components stored as files within it. You cannot simply rename the file to get it into the new format. To save the file in XSLX you'll have to save the file into the Excel 2010+ format.
If you're using Excel interop then it is an option on the SaveAs method.
for more info check the function: _Workbook.SaveAs Method
and the property: FileFormat:
Optional Object.
The file format to use when you save the file. For a list of valid choices,
see the FileFormat property. For an existing file, the default format is the
last file format specified; for a new file, the default is the format of the
version of Excel being used.
msdn info here:
https://msdn.microsoft.com/en-us/library/microsoft.office.interop.excel._workbook.saveas(v=office.11).aspx

reading embeded pdf files from excel using c#

I have an excel sheet which is embeded with PDF files. Is there any way to read that embeded PDF files from excel work sheet and saving in to data base using C#.

I believe that u can find some classes in Microsoft.Office.Interop.Excel namespace which can help to extract object embedded into Excel sheet.
for example WorksheetClass class

Need to stream Excel file to web clients

I have a method that converts a SQL query into a DataView and now I'm trying to convert a DataView to an Excel document that can be streamed from an asp.net site.
I've tried generating a .csv file but Excel would mess up the format.
Then tried OpenXML it appears super slow.
I just need to write out some basic table data so it can be modified in Excel and re-imported.

For creating an excel file you could use the Infragistics Excel engine. There is an example of how to create an excel file from a DataTable here: http://help.infragistics.com/NetAdvantage/ASPNET/Current/CLR3.5/?page=ExcelEngine_Populating_a_Worksheet_from_a_DataSet.html
You could then load the data again from the excel file format as well. If you want to test this you could do so with a trial of NetAdvantage for ASP.NET if you don't already have this:
http://www.infragistics.com/dotnet/netadvantage/aspnetdownloads.aspx

Generating Excel file from HTML

I'm aware that I can generate an HTML <table> and save it as an .xls file to read into Excel, and that works fine and all, but it only gives me one sheet.
Is there a way to generate HTML so that I can have multiple sheets in a single .xls file? I've tried to simply generate more then one <table>, but they just end up getting appended to eachother.

Short Answer: No.
Longer Answer: You cannot cause an HTML generated page to split into multiple worksheets in an Excel file. Further, the HTML you generate for even a single page could cause Excel to choke on certain machines as it does the conversion when the file is loaded. We've seen a number of low powered machines take upwards of 5 minutes to show a HTML file in excel (simple table with rows/columns, nothing fancy) depending on size.
Better Answer: Use a third party product like ClosedXML or FileHelpers to generate a proper xlsx file.

there seems to be way though I didn't try it, see http://www.c-sharpcorner.com/UploadFile/kaushikborah28/79Nick08302007171404PM/79Nick.aspx and check the Worksheets attribute
check the official documentation at http://msdn.microsoft.com/en-us/library/Aa155477%28office.10%29.aspx
After installing the Help file you can find an example of a file with 3 Worksheets in XML Reference / Excel Workbook...

You can use open source ClosedXML, а wrapper around OpenXML to conveniently generate xlsx files - i.e. Office 2007+ format Excel files.

Convert XLS to XSLX with Sharepoint and C#

I know about Word Automation Services, where I can start a ConversionJob which enable you to convert a .docx file to e.g. PDF or .doc.
I thought these services also allow the conversion of XLSX to XLS file - I was wrong. When looking at the SaveFormat Enumaration it only supports Word. Excel Automation Services don't seem to have such a conversion job?
How do I convert an XLS file to XLSX without using Excel automation (i.e. without having Excel installed on the server)?
EDIT:
In the end I used Aspose Cells for the conversion.

You might try ExcelLibrary or EPPlus those libraries allow you to write excel files without using excel COM object.
You may read cell by cell and create a new worksheet copied from the other one (copy cell by cell)

I'm note sure that you can do it (converting XLS with is a row MS Excel file without having either Excel Services or MS Excel installed [Using InterOp] to XLSX which an OpenXML format)!!
If you want a way to convert it on a PC which MS Excel install check this link out http://devville.net/blog/2011/02/05/how-to-convert-rtf-document-to-doc-using-c/
But if found a way I would be happy if u share it to use.