The Sitecore Content Search API uses the native Microsoft Windows IFilter interface to extract the text content from media files for indexing.

However, to enable the Sitecore Content Search API to properly index the content in Adobe PDF files, you must install the Adobe PDF IFilter on every content management and content delivery server.

If you failed to install IFilter on your Sitecore instance then you will get a warning in your crawling log. This warning will be logged for each pdf items you are trying to index (In my case each crawling log was of 500 MB size so the disc’s usable size was shrinking very fast)

WARN Could not compute value for ComputedIndexField: _content for indexable: sitecore://web/{B9B8870D-3504-908F-42BC-BD2843229C3D}?lang=en&ver=1
Exception: System.Runtime.InteropServices.COMException
Message: Exception from HRESULT: 0x80048605
Source: Sitecore.ContentSearch
at Sitecore.ContentSearch.Extracters.IFilterTextExtraction.IPersistStream.Load(IStream stream)
at Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterLoader.InitializeFilterAsPersistStream(IFilter filter, String fileName)
at Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterLoader.LoadAndInitIFilter(String fileName, String extension)
at Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterReader..ctor(String fileName)
at Sitecore.ContentSearch.ComputedFields.MediaItemIFilterTextExtractor.ComputeFieldValue(IIndexable indexable)
at Sitecore.ContentSearch.ComputedFields.MediaItemContentExtractor.ComputeFieldValue(IIndexable indexable)
at Sitecore.ContentSearch.LuceneProvider.LuceneDocumentBuilder.<>c__DisplayClass12_0.<AddComputedIndexFieldsInParallel>b__0(IComputedIndexField computedIndexField, ParallelLoopState parallelLoopState)

To fix this issue you have to install Adobe PDF IFilter version 9 on every instance of your Sitecore installation. So download IFilter from  ftp://ftp.adobe.com/pub/adobe/acrobat/win/9.x/

and install. Once you install and reset your IIS check your crawling log and make sure the warn not registering anymore. For more details refer  Sitecore Documentation